A SHELF SUPPORT ASSEMBLY FOR A REFRIGERATOR UNIT FIELD OF THE INVENTION The present invention relates to a shelf support assembly for a refrigeration unit. In particular, the invention relates to a shelf support assembly for a commercial refrigeration unit such as a refrigerator, freezer or other cold display cabinet. The present invention also relates to a method of constructing a refrigeration cabinet. BACKGROUND OF THE INVENTION The present invention will be described with particular reference to commercial refrigeration cabinets such as those used in convenience stores, takeaway shops, supermarkets and the like. However, it will be appreciated that the present invention may find application in any suitable type of refrigeration unit, including domestic refrigeration units and no limitation is intended thereby. Commercial refrigeration and freezer cabinets have swinging insulated glass or solid doors mounted on the cabinet frame. Vertical members known as mullions extend from the floor to the top of the cabinet frame so as to provide a stop for the swinging sides of the doors when they are closed. Importantly, the mullions also provide a sealing surface for creating a sealing surface against which the door gaskets may seal when they are closed. The combined weight of the insulated glass or solid doors and a fully stocked refrigerator can be considerable. The mullions are therefore also designed to provide strength and rigidity to the refrigeration cabinet frame. The mullions must also be strong enough to resist the forces to which they are subjected when the doors are being opened and closed or slammed. In a commercial environment, the doors may be opened and closed many times and often without care by customers and/or staff. Mullions are constructed of a metallic material such as steel or aluminium. As aluminium is non-magnetic and cannot attract a magnetic door seal, it is common practice to mount a magnetic steel sealing plate to the front of the mullion. It is also common practice to mount a fluorescent light to the rear of the mullion for display purposes. 1 Aluminium and steel are highly thermally conductive which means that that part of the sealing plate that is exposed to the atmosphere between the edges of the doors can be cooled below the dew point of the ambient air. This causes condensation on the sealing plate which is at best unsightly and at worst may lead to mould growth and rusting. It is therefore, also well known to electrically heat mullion assemblies so as to prevent condensation. However, providing mullion heaters adds to power costs. With a view to addressing the condensation problem, there have been many arrangements designed to thermally insulate components of a mullion assembly and in particular to insulate the sealing plate from the refrigeration zone. Thus some refrigerators do not have a mullion heater, although they are generally provided for freezer cabinets. Thus it may be appreciated that a mullion assembly can comprise a number of parts including the main structural member, a steel mounting plate, heating elements, a light tube and associated wiring. There are also a number of methods of attaching the structural mullion member to a refrigeration cabinet frame. One method simply uses an L shaped connector bracket to connect a mullion of square or rectangular cross section to the frame. As the front of the mullion must sit flush against the front of the cabinet so as to provide a seal against the doors, this limits the bracket to being mounted to the rear of the mullion. This method requires separate installation of a mullion light after the mullion has been installed in the refrigeration cabinet. This requires the light assembly with its associated electrical wiring to be mounted to the rear on the main mullion. Structure after the mullion has been fitted to the cabinet. This is time consuming and difficult in that the light tube is mounted to the rear of the mullion inside the cabinet. This can increase the costs of construction. Another method for mounting a mullion uses a mullion of rectangular cross section having vertically extending tabs in the form of extending sections of the front and rear faces of that are dimensioned to locate over the upper and lower sections of the refrigeration cabinet 2 frame. This arrangement permits heat transfer into the refrigerated space though the metal mullion. Further as the mullion face overlaps the cabinet face, this has an undesirable uneven surface for the doors to seal against and has a less appealing appearance looking somewhat of an afterthought mounting. It is also desirable that the mullion mounting provides a join that is neat in appearance and provides an easy to clean surface. It is clearly undesirable for the fixing or mounting method to provide spaces or recesses in which dirt, debris and/or moisture may collect. One known arrangement for mounting a mullion to a refrigeration cabinet frame that seeks to address the above difficulties provides a mullion that is an extruded aluminium or plastic member having a profile that enables a cover strip to be snapfitted thereto. The cover strip is made from a thermally insulating material to avoid condensation. Magnetic strips are mounted between the cover strip and the mullion to magnetically attract the doors. The profile also has a channel for receiving a plate to which a fluorescent light tube can be mounted. In this way, the assembly may be installed as a single unit. The assembly is mounted to the cabinet frame by means of plastic end shoes that fit over each end of the mullion. The end shoes have a mounting bracket portion that can be screwed to a cabinet frame. An alternative method of mounting a mullion to a refrigeration cabinet is described herein. Commercial refrigerators having two or more swinging doors generally have a vertical shelf post support located behind the mullion. The base of the post is fixed to the floor of the refrigerator space which is usually a steel sheet. Generally, the support has a series of apertures for receiving shelf clips for mounting the shelves such that customers can adjust the spacing's between shelves. The opposite sides of the shelves are supported by like vertical supports located at the sides of the cabinets for 2 door refrigerators or onto another vertical support located being a second mullion on a 3 door refrigerator. A problem with shelf supports located behind mullions and intermediate the side walls of a refrigeration cabinet is that a significant amount of weight is transferred to the cabinet floor. This is not a significant problem where the shelf supports are located adjacent the side walls 3 as the weight is transferred to the side supporting members. On the other hand there is no vertical support located below the mullions. It is often impractical or not possible to provide such support, especially for refrigerators having a bottom mounted refrigeration package. A bottom mounted package is desirable for ease of servicing and they are generally designed to be easily replaced. A consequence of a lack of support is that the weight carried by the shelf support is transferred to the floor which can cause buckling or distortion of the floor. This in turn can cause a break in the seal against the doors so as to allow heat transfer into the refrigerated space. For cabinets having bottom mounted refrigeration packages, buckling of the floor can also comprise the space below the cabinet's insulated floor making the refrigerator package difficult to move and replace. In order to address this problem, it is known to provide a horizontal support bar formed from galvanised steel that extends along the metal floor of the cabinet upon which the base of the shelf support post is mounted so that the weight is distributed across the floor. Such a solution is accepted in the industry. As mentioned above bottom mounted refrigeration packages are advantageous in that they provide easy service access. Another advantage is that hot air from the unit may be directed upwards along the face of the glass doors to avoid condensation. However, having the unit below the floor means that heat generated by the unit may be transferred into the refrigeration space. Accordingly, insulation is provided between the metal cabinet floor and the refrigeration space. There is also disadvantages with such a known solution for refrigeration packages having top mounted refrigeration units or packages. The present inventor has surprisingly found that by modifying the design of the support bar that heat transfer from a bottom mounted refrigeration unit or packages into the refrigerated space can be reduced. This increases the efficiency of the refrigeration unit which translates to a decrease in operating costs. The modified support ban also provides advantages for refrigeration cabinets having top mounted units or packages. Top mounted models usually 4 have the castors attached directly to the bottom of the cabinet and the support bar would span between the castors. Sometimes an external support bar is attached to the bottom of the cabinet and the castors mounted to it but by doing this it increases the height of the cabinet or reduces the internal height to keep the cabinet to an acceptable external height. With an external support bar there is only the foam insulation supporting the internal floor of the cabinet and can compress the foam causing a depression in the internal floor. This causes the shelf support post to drop putting the shelving out of level. It is therefore an object of the present invention to provide an alternative floor support bar arrangement for a refrigeration cabinet and in particular a cabinet having a bottom mounted refrigeration unit. SUMMARY OF THE INVENTION According to a first broad form of the invention, there is provided a shelf support assembly for a refrigerator cabinet having opposed side walls and a floor, the shelf support assembly including a vertical shelf support post having a base at a position intermediate the opposed side walls, an elongate support member formed from a material having a thermal conductivity of less than about 30W/m-*K the elongate support member being mounted to the floor transverse to the side walls, wherein the base of the shelf support post is supported by the elongate member and a spacer is located between the base of the shelf support post and the elongate support member, the spacer having a thermal conductivity of less than about 5W/m -*K. Suitably, the refrigerator cabinet has a bottom section beneath the floor for housing a refrigeration package. The elongate support member has a thermal conductivity of less than about 30W/m-*K, preferably less than about 25 W/m-*K, most preferably less than about 20 W/m-"K. Most carbon steels have a value higher than this. For example, conventional reinforcement members are formed from galvanised steel having thermal conductivity values of between about 36 to about 54W/m-K*. A preferred material is stainless steel and in particular 304 stainless steel that has a thermal conductivity of about 16.3W/m-*K. 5 The spacer is made from a material that has a low thermal conductivity of less than about 5 W/m-2K, preferably less than about 2.5 W/m-2K and most preferably less than about 1 W/m QK. Suitably the spacer is formed from a substantially non-compressible, by substantially non compressible, it is meant that the material will not appreciably compress or deform under the load of a fully stacked refrigerator. Suitable low conductive materials include plastics, such as nylon (conductivity 0.25W/m-2K), high density polyethylene (conductivity 0.50W/m-2K), glass fibre epoxy (conductivity 0.23W/m-2K), ceramics . Alternate materials are engineered wood product materials. A preferred material is marine ply (conductivity 0.13). The elongate support member is suitably formed in a hat section. Suitably the space between the floor and the base of the shelf support post is filled with an insulating material as is known in the refrigerator art. The support member and spacer is typically mounted to the floor and the insulation applied as a foam. Suitably, the support has a series of holes through which the foam may pass. After the insulation has cured, the support member will be completely encased within the insulation with the top of the spacer available to have the base of the shelf support post mounted thereto. Broadly described herein is a mullion assembly for mounting to a frame of a refrigeration cabinet, the assembly comprising an elongate structural member having a first surface that in use is a front facing surface and an opposed rear facing surface, an elongate strip member engaged with and along the length of the rear surface of the structural member, the strip member having a mounting tab at each end for mounting the assembly to the cabinet frame. The structural member is suitably a plastic extrusion having a profile. The profile is typically a generally C shaped cross section having a closed front face and an open rear face defined between the arms of the C. The structural member can alternatively be an aluminium extrusion. Plastic or other thermally insulating material is preferable however to reduce thermal transfer. 6 The front face of the mullion may be adapted to receive a cover strip of a thermally insulating material and/or a magnetic cover plate as known in the art. The mullion may be configured such that the cover strip may be snapfitted thereto. The front face of the mullion may be provided with an access aperture suitably in the top section thereof for providing access to electrical wiring. In use, the access aperture may be sealed by the cover strip, if present. As the strip is snapfitted to the mullion, it may be removed to easily expose the access aperture so as to provide access to wiring without requiring the whole mullion assembly to be removed. The elongate strip member is engaged with and along the length of the rear facing surface of the structural member. The strip member may have a profile that is complimentary to a section of the structural member. Where the structural member is generally C shaped in cross section, the strip member may also have a generally C shaped cross section in which the opposed arms are received within the profile of the structural member. Suitably the arms of the structural member have a return lip for retaining the strip member. When engaged in this manner, the main body of the strip is spaced from the front of the structural member to define a space therebetween. The strip member has mounting tabs at each end thereof. The tabs suitably extend at generally right angles to the main body of the strip member. The tabs suitably extend to the rear of the structural member. The tabs may be mounted to the cabinet frame by any suitable means. Suitably the tabs are provided with apertures for receiving screws or other fastening means. The strip member may have a light mounted thereto. Preferably the strip is in the form of a modified light batten. The modified strip allows the light fittings and associated electrical wiring to be fitted to the strip, eliminating the need for a separate light batten. In the embodiment where the strip is a light batten, the structural member may be adapted to receive and engage a light diffuser. Suitably the structural member has recesses for receiving edges of the light diffuser in snap or friction fitting engagement. 7 The light diffuser may extend the full length of the mullion. Alternatively the diffuser may terminate short of the top and bottom ends of the mullion. In this case, it is preferred that the assembly further include one or two U shaped end caps or cover that respectively fill the space between the top and/or bottom edges of the light diffuser and the top and bottom portions of the cabinet frame to which the mullion is attached. The U shaped cap(s) are suitably fitted after the mullion has been installed. The U shaped caps suitably have arms with terminal ends configured to be received within corresponding recesses in the mullion for friction or snap fitting engagement. The caps may also have means for fixing the caps to the cabinet frame. Suitably the web of the U shape is adapted to be screwed or otherwise fixed to the frame. This provides additional support to the mullion. Also broadly described herein is a method of manufacturing a mullion assembly for mounting to a frame of a refrigeration cabinet, the method comprising providing an elongate structural member having a first surface that in use is a front facing surface and an opposed rear facing surface and engaging an elongate strip member with and along the length of the rear surface structural member, the strip member having a mounting tab at each end for mounting the assembly to the cabinet frame. The present invention also relates to a refrigerator cabinet including the shelf support assembly that optionally includes the mullion assembly as described herein. BRIEF DESCRIPTION OF THE FIGURES Figure 1 is a perspective view of a refrigerated cabinet having a mullion assembly; Figure 2 is a perspective view of a preferred mullion assembly of the present invention; Figure 3 is a schematic view of the upper section of the mullion assembly shown in Figure 2 of the present invention prior to installation of the end cap; Figure 4 is the same view as shown in Figure 3 with the end cap in place; 8 Figure 5 is a schematic perspective view of a floor section of a refrigerator cabinet showing a preferred floor support arrangement of the present invention; Figure 6 is a further schematic perspective view of the floor section as shown in Figure 6; Figure 7 is a schematic side on view of the arrangement as shown in Figure 5 and Figure 8 shows a perspective view of the removal of the light diffuser section of the assembly as shown in Figure 1. Figure 1 shows a commercial refrigeration cabinet 10. The cabinet 10 has a frame 12 upon which are mounted two swinging glass doors 14, 16. The doors close and seal against a central vertical mullion assembly 18. The assembly 18 includes a light tube 20 mounted to illuminate the products on display. The light tube 20 is suitably an LED tube so as to reduce energy consumption. A refrigeration package is installed in the bottom of the cabinet below the floor of the refrigerator space. A removable front grill 21 is provided to allow access to the condenser for cleaning or to remove and clean a filter (if provided). Figure 2 is a perspective view of the mullion assembly 18. The assembly has a structural member 22 that is an extruded plastic mullion member. The light tube 20 is mounted behind the mullion 22 and is surrounded by a light diffuser 26. Figures 3 and 4 show a detail of the top part of the mullion assembly 18. The extruded plastic mullion 22 has a generally C shaped profile. The profile has a front face 27 with rearwardly facing opposed side arms 28, 30. Each arm terminates in an interned lip 32. A plastic strip 32 is snap fitted to the front face 27 of the mullion 22. The plastic strip 32 is formed from a thermally insulating plastics material such as PVC. A pair of magnetic strips may be placed between the plastic strip 32 and the front face in elongate channels 33 defined between the plastic strip 32 and the front face 27 of the mullion 22. As the plastic strip is not thermally conductive, that part of the strip that is between the edges of the doors and in contact with the atmosphere does not cool and attract condensation. The light tube 20 is mounted on a metal light batten 36 which is in the form of a C shaped strip member having a flat rear face and a front open face defined by opposed arms 38, 40. 9 The batten 36 is configured to be complimentary to that of the C profile section of mullion 22 so that the two can be slidably engaged. Such slidable engagement allows for ease of assembly. The front face of the mullion 22 and the rear face 43 of the batten 36 have a communicating opening 41 through which electric wires can pass for the light and if present a mullion heater. Generally, heating would only be required where the unit is a freezer. The opening 41 is closed by the front face 27 of the mullion. This arrangement will be described in further detail below. Recesses 45 are provided in the length of the mullion 22 to retain a heater wire and for transferring heat to the mullion 22 and face 32 for anti-sweat purposes where required. Each end of the batten 36 has a rearwardly projecting tab 42 or tongue. The tabs are formed from a bent over end of the batten. The tabs 42 have two apertures 44 for receiving screws or other fixing means for mounting the top and bottom of the mullion assembly to the refrigerator cabinet frame. There is a degree of flexibility about the bend line that can accommodate slight movement that may occur during installation and servicing. It will be appreciated that the use of a separate bracket is not required to secure the mullion assembly to the frame. This reduces assembly and installation time and thus manufacturing costs. The light diffuser 26 does not extend fully to the top and bottom terminal ends of the mullion 22. As shown in Figures 3 and 4 a U shaped end cap 50 with opposed legs 52 with barbed ends snap fits into the top of mullion 22. An identical cap fits to the bottom of the mullion 22. The end caps 50 provides a neat and flush appearance to the mullion assembly. The end caps 50 have apertures 54 to allow them to be screwed or otherwise fixed to the cabinet frame. This further secures the assembly in place. The end caps are installed after the main body of the assembly has been screwed to the cabinet frame. The end caps 50 are then installed followed by snapfiting of the diffuser to the mullion. 10 Figure 8 shows the diffuser 26 being removed by insertion of the end of a screwdriver 61 between the diffuser and the mullion 22. It may be appreciated that providing a mullion assembly having the elongate member insertable within the profile of the plastic mullion provides some advantages in fixing a light tube to a mullion. However, it may also be appreciated that this limits the locations at which the mullion may be attached to the frame. As discussed in the background portion of the specification, an earlier approach has been to provide a rectangular shaped plastic end cap that mounts over the entire top of the whole assembly including the front plastic strip. The end cap has a mounting portion that is screwed to the cabinet frame. These end caps mount the mullion and have to be in place when fitting and removing the mullion. This mounting being plastic is not strong and this mounting method makes it difficult to fit and remove the mullion. However, the present mullion assembly has a number of hitherto unrecognized advantages over this prior arrangement. First, the mounting point is towards the centre of the mullion assembly and the tabs used to mount the assembly are metal. This provides a much stronger and more positive method of attachment. As mentioned, centre mullions of commercial refrigerators may be subject to abuse over many years of their working life. Any movement of the centre mullion can adversely affect the sealing between the door gasket and the mullion. This requires the refrigeration unit to operate for longer periods so as to maintain a desired temperature and thus increases running costs. Fitting of the end caps 50 after the mullion assembly has been fitted makes fitting of the mullion assembly easier and makes the mullion electrical wiring easier to connect to the main cabinet electrical. Figures 5 to 7 show schematic views of a section of the floor of the refrigerator cabinet as shown in figure 1. The centre mullion 18 is shown, behind which is a shelf support post 70. The post has a series of spaced apertures 72 for receiving shelf support clips in a manner know in the art. For illustrative purposes the insulation floor layer of polyurethane foam is not shown so that the metal outer sheet of the floor section 74 of the cabinet may be seen. 11 Beneath the floor 74 is a box section 76 that in use houses a refrigeration package or unit. The base of the box section is mounted on castors 78. It will be appreciated that in view of the location of the refrigeration unit below floor 74 that heat generated by the unit will be transmitted to floor 74. Thus proper insulation is necessary. An elongate support member 80 being in the form of a 304 grade stainless steel hat section is mounted to the metal floor 74. The section has a series of holes 81 through which the insulation flows as discussed below. Between the base of the support post 70 is a spacer 82 formed from marine ply. Marine ply has a low thermal conductivity of about 0.13W/m-K2. Below each end of the elongate member 80 are two vertical supports 84 extending below the floor 74 to the top of the front plate 86 upon which the castors 78 are mounted. This distributes the weight of the shelf to the castors 78. Figure 7 shows a side view in which the level of the floor insulation is shown by reference numeral 84. The insulation reaches the top of the spacer 82 and completely encapsulates the support member 80. This is advantageous when compared with the prior art arrangement in which the base of the shelf support post is screwed directly into the support. It is necessary for there to be a surface above or level with the top of the insulation in which to screw the base of the shelf post. However, exposure of the top of a metallic support member increases the transfer of heat from the refrigeration unit into the refrigerated space. Although making the support member from stainless steel which has a lower thermal conductivity than other metals used in refrigerator construction, the combination of the use of stainless steel and the spacer in combination enables a surprising reduction in heat transfer which translates to energy savings. It will be appreciated that such a method of providing support to a shelf support post can minimize heat transfer from the refrigeration unit to the refrigerated space thereby improving efficiency and thus running costs. It will be appreciated that various changes and modifications may be made to the present invention as described and claimed herein without departing from the spirit and scope thereof. 12