ULTRA-VIOLET DISINFECTION APPARATUS This invention relates to ultra-violet disinfection, particularly but not necessarily exclusively in relation to treating effluent, waste water, including secondary effluent prior to discharge to the environment, industrial waste water, as well as potable water. 5 Background it has been a well known and commonly used technique to disinfect effluent or other water products by exposing the water to ultra-violet radiation of a wavelength suitable to kill or inactivate micro-organisms, particularly pathogenic organisms. The exposure to the water requires UV radiation of a suitable intensity and for a suitable 10 period of time to achieve the disinfection. This irradiation by ultra-violet light is known to be performed within a liquid vessel which may be a treatment tank, or treatment channel or passage in which the liquid is contained for a suitable residence time (which may include time during which the liquid takes to pass through the vessel). The ultra violet radiation is generated by UV sources immersed in the liquid vessel. 15 One known type of UV disinfection apparatus includes multiple UV radiation lamps usually in tubular form similar to fluorescent visible spectrum lamps, except that the UV lamps have quartz casings or housings so as to transmit the UV radiation (since conventional glass is opaque or at least significantly attenuates UV radiation). The UV lamps are also usually enclosed with quartz sleeves forming part of the mount enabling 20 individual UV lamps to be isolated from contact with the water being treated and facilitating handling of the lamps for their periodic replacement for example. The quartz sleeves are part of the mount system for the UV lamps and themselves need periodic cleaning. Such UV disinfection apparatus also includes a structure associated in use with the 25 liquid vessel in which the liquid to be disinfected is located. The liquid vessel for example can be a tank or channel or a passage through which the liquid flows whilst being subjected to the UV radiation flux. The structure is arranged to locate and support the mount for the UV radiation sources. In such apparatus it is known that the mount can be lifted from an operative position immersed in the liquid in the vessel by being hoisted 30 upwardly to a lifted position enabling access to the mount and to the UV sources for cleaning, servicing, maintenance, etc. Such arrangements for lifting the mount out of the liquid vessel can be complex and relatively expensive because they use overhead support systems, such as gantries, travelling winches for movement on the gantries, and apparatus for coupling to and lifting the mounts out of the vessel. This can also make them moderately complex and/or slow and inefficient to operate. It is an object of the present innovation to provide an improved UV disinfection apparatus enabling ready movement of a mount for UV radiation sources between an 5 operative position immersed in a vessel 'for liquid being treated and a retracted position enabling access for servicing or the like. Summary of the Invention According to the present innovation there is provided an ultra-violet disinfection apparatus including: 10 a mount constructed and arranged for a plurality of UV radiation sources to be removably supported by the mount, and a structure operatively associated in use with a liquid vessel in which liquid to be disinfected is located, the structure being arranged to locate and support the mount so that the mount is selectively movable between an operative position immersed in the liquid in 15 the vessel and a retracted position in which the mount is lifted out of the vessel and is accessible for cleaning, servicing, maintenance, or the like, wherein the apparatus includes an articulated connection between the mount and the structure so that the mount moves angularly about the connection in moving between its operative position and its retracted position, and 20 wherein the apparatus includes a lifter operative to apply a lifting force to assist lifting of the mount when moving between its operative position and its retracted position. Preferably the lifter comprises a passive lifting mechanism operative to apply the lifting force without requiring activation by a user initiating movement of the mount from 25 its operative position to its retracted position. Preferably the passive lifting mechanism is operative to apply a biasing force to the mount in a direction to assist lifting of the mount in movement between its operative position and its retracted position, the biasing force being insufficient to move the mount between its operative position and its retracted position without application of additional 30 force to effect such movement of the mount and being sufficient to apply significant force to assist that lifting movement and reduce the amount of additional force required. In a preferred embodiment the lifter includes a gas strut or spring operatively connected to apply lift assisting force to the mount when being moved from its operative 2 position to its retracted position, the gas strut or spring being in its compressed condition when the mount is in its operative position. The articulated connection between the mount and the structure preferably comprises a pivot connection. The pivot connection is preferably arranged so that the 5 mount pivots about a horizontal pivot axis and remains in a generally vertical plane when moving from its operative position to its retracted position. In a preferred embodiment the pivot connection is located at an upper corner of the mount when the mount is in its operative position so that the mount lifts out of the liquid vessel and upon reaching its retracted position is located at least partially laterally displaced relative to its operative 10 position in the liquid vessel thereby facilitating user access to the mount. One preferred arrangement provides a mount which when located in its retracted position is substantially inverted relative to its operative position. Brief Description of the Drawings Possible and preferred features of the present invention will now be described with 15 particular reference to the accompanying drawings. However it is to be understood that the features illustrated in and described with reference to the drawings are not to be construed as limiting on the scope of the invention. In the drawings: Fig. 1 is a schematic view of apparatus according to one embodiment of the invention showing its operation, and 20 Fig. 2 is a schematic perspective view of a water disinfection plant utilising the present invention. Detailed Description of Preferred Embodiments The apparatus shown in Fig, 1 is for disinfection of liquid, such as waste water, in a liquid vessel (11) schematically illustrated in cross-section. The liquid in use is resident 25 in the vessel (11), for example by flowing through the vessel from an inlet to an outlet and, while in the vessel (11), the liquid is exposed to an ultra-violet flux for disinfection purposes. The apparatus includes a mount (20) and a plurality of UV radiation sources (25) removably supported by the mount. The mount (20) and UV sources (25) form a 30 generally planar assembly which in the illustrated apparatus is generally vertical when in its operative position immersed in the liquid in the vessel (11). The mount includes for example a head (21) and spaced arms (22, 23) extending downwardly from the head (21) and between which the UV radiation sources (25) shown as elongated tubular lamps 3 (which would be enclosed within quartz sleeves) are arranged. Electrical supplies to the UV sources (25) are located within the head (21) and arms (22, 23). The apparatus also includes a structure (30) associated with the liquid vessel (11). In Fig. 1. the structure (30) includes a body (31) at one side of the vessel (11), the body in 5 the drawings comprising an upright post or the like. The structure (30) locates and supports the mount (20) in a way that the mount is selectively movable between the operative position shown at "A" immersed in the liquid in the vessel (11) and a retracted position shown by the letter "D" in which the mount is lifted out of the vessel and is accessible for cleaning, servicing, maintenance etc. 10 An articulated connection (35) is provided between the mount (20) and the structure (30), in particular the top of the body or post (31), so that the mount (20) moves angularly about the connection (35) in moving between its operative position A and its retracted position D, passing through the intermediate positions marked "B" and "C". The connection is shown as comprising a pivot connection (36), the pivot connection 15 being arranged so that the mount (20) pivots about a horizontal pivot axis and remains in a generally vertical plane when moving between its operative position A, through its intermediate positions B and C, and its retracted position D. The pivot connection (36) is located generally at an upper corner of the mount (20) when the mount is in its operative position A. In Figure 1, the pivot connection (36) is 20 located on an extension (24) of the head (21) so that the pivot point (36) is displaced laterally from the vessel (11). In this way, the mount (20), when being pivoted, lifts out of the liquid vessel (11) and upon reaching its retracted position D is located laterally displaced relative to its operative position A and this can facilitate user access to the mount for cleaning, servicing, maintenance, etc. As seen in Fig. 1, the mount (20) is 25 substantially inverted in its retracted position D and users can gain access to the mount from platforms, walkways, etc. located adjacent the vessel (I1) and in the vicinity of the mount (20) when in its retracted position D. Although the articulated connection (35) is shown as a pivot (36) so that the lifting and translation of the mount (20) to its retracted position D is a pivoting movement about 30 the axis of the pivot (36), this kind of articulation is not necessary to the invention. For example, a suitable linkage arrangement could be provided so that the movement of the niount from its operative position A to its retracted position D involves an initial generally linear upwards movement raising the mount in a direction to lift it out of the 4 vessel (11) and, when the mount (20) has lifted by a predetermined degree but not necessarily completely out of the vessel (1. 1), the linkage arrangement can then effect angular movement similar to the rotational movement described above and illustrated in Fig. 1. 5 The apparatus also includes a lifter (40) which is operative to apply a lifting force to assist lifting the mount (20) when moving between its operative position A and its retracted position D. The lifter (40) is a passive lifting mechanism so that it applies a lifting force without requiring activation by a user initiating movement of the mount (20) from its operative position A, e.g. without requiring activation by user operation of an 10 electrical drive to apply its Ii fling force. In the illustrated preferred embodiment, the lifting mechanism (40) applies a biasing force to the mount (20) in a direction to assist lifting of the mount, the biasing force being insufficient to move the mount (20) from its operative position A and towards its retracted position D without application of additional force to effect such movement, but nevertheless being sufficient to apply 15 significant force to assist that lifting movement and reduce the amount of additional force required. For example, the initiation of the movement of the mount from its operative position A to its retracted position D, and the application of the additional force beyond that applied by the lifting mechanism (40) may be a manual lifting operation. For example, the upper head (21) of the mount (20) may be located above the level of the 20 liquid in the vessel (11) and may be provided with lifting handles or other engageable components by which a user may manually apply a lifting force. With the manually applied lifting force, in conjunction with the significant assisting force applied by the lifting mechanism (40), the mount (20) may be capable of being lifted out of the vessel (11) without excessive or substantial lifting force by the user being required, hence 25 potentially reducing occupational health and safety dangers that might otherwise exist. In the preferred embodiment, the lifter (40) comprises a gas strut or spring connected so ns to annly lift assistinp forces to the mount (2) when heing nioverl from its operative position A to its retracted position D. The gas strut (41) is in its compressed condition when the mount is in its operative position A as shown in Fig. I. As the mount 30 (20) is lifted to the intermediate position B and up to a position when the head (21) will be substantially vertical, the gas strut (41) continues to apply lifting force, although from positions such as position 13 to the vertical position the lifting force is progressively reducing to zero at the vertical position. However less lifting force is required during 5 those stages of movement of the mount (20) so that the progressive reduction of the lifting force applied by the gas strut (41) does not significantly increase the force required to be applied by the operator. When the head (21) passes beyond the vertical and towards the retracted position D. passing through the intermediate position C. the gas 5 strut applies progressively increasing force resisting the movement of the mount (20) downwardly towards its retracted position D, thus helping to support the weight thereof. By providing the gas strut pivotally mounted at both ends located to and extend between the structure (30) and the mount (20), the gas strut (41) assists lifting of the mount from the vessel (11), assists lowering of the mount to its retracted position D, and then again 10 assists lifting of the mount (20) from its retracted position D when being returned to its operative position A, passing through the intermediate positions C and D. The gas strut may be of conventional construction and operation. Testing with a mount of conventional construction and dimensions with multiple UV radiation rubes a gas strut having a compressed gas force of 600 Newtons has been found suitable. Of course, the 15 force applied by the pressurised gas can be chosen to suit the weight of the mount and to suit the mechanical advantage required depending on the dimensions and action points at which the gas strut is operating. In Fig. 2 there is a water treatment vessel (11) (shown with its near wall cut away) and within the vessel is an array or bank of n-ounts (20) each having multiple LV sources 7( (25), Each mount (20) is capable of hein individually moved from its operative position shown in Fig. 2 by being rotated about its respective pivotal mounting (35) to a retracted position enabling clear access to the mount and the IV sources. The lifter strut associated with each of the mounts is not shown in Fig. 2 but will be located and operative substantially as described above in relation to Fig. 1, 25 it will be seen that the apparatus described herein and illustrated can be relatively simple in construction and operation and need not be complex or expensive to produce, install and maintain. The mounts for the I sources can he quickly and easily moved out of the water irealment vessel for maintenance and servicing. The lifting force needing to be applied or exerted by a user or operator can be reasonably modest and well 30 within acceptable occupational health and safety guidelines. It is to be understood that various alterations, modifications and/or additions may be made to the features of the possible and preferred embodiment(s) of the invention as herein described without departing from the spirit and scope of the invention. 6