1 POST AND BEAM SUPPORT BLOCK This invention relates to a post and beam support block. The post and beam support block may be referred to as a grade foundation block. In this specification, the term "grade 5 foundation" refers to a foundation in respect of which no substantial earthworks are performed or if earthworks are performed, then such works are limited to plateau-type excavations or to provide substantially flat areas of ground upon which the grade foundation block may be supported. LO Grade foundation blocks formed from concrete or light weight concrete have been provided for supporting structures above grade or on-grade foundations. Typically, the ground is prepared to be substantially flat and adequately compacted to form the on-grade foundation. Although such blocks provide for L5 support of a post, or a beam for a bearer or joist, they are normally limited to providing for only one size of post or beam. Due to the limitations of the mechanical properties of concrete and light-weight concrete, grade foundation blocks formed therefrom have a minimum height which normally requires 20 that the height of the floor above the joists is at least two standard size steps above the grade of the foundation. If the ground is sloping, then shortest support post for the structure supported by the grade foundation blocks must be at least the minimum height. 25 In this specification, terms such as upward, downward, horizontal and vertical, and their grammatical derivatives, are used to describe the invention in its normal orientation, and are not to be construed to limit the invention to any particular orientation. 30 The present invention aims to provide a post and beam support block which alleviates the aforementioned 2 shortcomings. Other aims and advantages of the invention may become apparent from the following description. With the foregoing in view, in one aspect the present invention resides broadly in a post and beam support block 5 including: a platform; a side wall extending angularly from the edges of the platform, the side wall having one or more lower wall portions extending below the platform to provide an open lower cavity LO and a plurality of upper wall portions extending above the platform wall to provide an open upper side; and a base wall extending outwardly from the lower edges of the lower wall portion and having sufficient surface area for supporting a predetermined load upon a grade foundation; L5 wherein the edges of the platform have a configuration such that the upper wall portions extending therefrom are spaced from one another to provide one or more channels sized to accommodate a beam laying across and upon the platform between 20 at least part of the upper wall portions and to also provide one or more sockets intersecting with the or each channel, the socket or sockets each being sized to accommodate a support post upon the platform and extending upwardly therefrom between at least part of the upper wall portions, 25 the one or more lower wall portions being positioned in juxtaposition below each of the upper wall portions so as to support the platform in spaced disposition above the base wall. Preferably, a plurality of buttresses extend between one 30 or more of the outer faces of the side wall and the upper face of the base wall. More preferably, the buttresses extend between the outer faces of the lower wall portions and the upper face of the base wall. In a preferred form, the 3 buttresses are in the form of buttress walls radiating outwardly from the lower wall portions. Preferably, the lower wall portions include a sloping wall portion and an upright wall portion, the sloping wall 5 portions extending downwardly and outwardly from the lower edges of opposing upper wall portions and the upright wall portions extending between the lower edges of the sloping wall portion and the upper face of the base wall. Preferably, the base wall has a peripheral edge and a plurality of internal LO edges such that the base wall, lower wall portions and platform are substantially contiguous whereby a two-part mould may be used to manufacture of the support block from a mouldable material such as, for example, an engineering plastics material. L5 Preferably, one or more arch supports extend between opposing inner faces of the lower wall portion. The arch supports are preferably in the form of arch walls having a curved lower edge extending between the inner faces of the lower wall portion. It is also preferred that the parts of the 20 lower wall portions which support the upper platform astride the channel or channels are joined at their distal edges by a slanting wall extending at an acute angle to the platform and the base wall. It is also preferred that the lower wall portions have one or more openings penetrating therethrough to 25 provide ventilation between the lower cavity and the outer face of the lower wall portion. Preferably, the openings penetrate the slanting wall. In such form, the openings are sized to prevent passage of vermin, such as, for example, termites, through the openings, but still provide ventilation 30 to avoid build-up of moisture in the lower cavity. In a preferred form, there are two channels passing through the sockets, effectively intersecting with one another within the sockets, at substantially right angles. In such 4 form, it is preferred that the spacing between the upper wall portions have a different spacing for each channel so that two alternative sizes of beam may be supported on the platform between the upper wall portions having the corresponding size. 5 In a similar fashion, upper wall portion surrounding at least part of each of the sockets is formed to accommodate two alternative sizes of support post. Smaller sizes of support beam or support post may be accommodated by including a spacer, shim or such like on one or both sides of the support LO beam. Smaller sizes of support post may be accommodated in a similar fashion. In order that the invention may be more readily understood and put into practical effect, an exemplary embodiment of the present invention will now be described with L5 reference to the following drawings, and wherein: Fig. 1 is perspective view of a post and beam support block according to the invention; Fig. 2 is a front elevation of the post and beam support block of Fig 1; 20 Fig. 3 is a side elevation of the post and beam support block of Fig. 1; Fig. 4 is a cross section of the post and beam support block taken online 4-4 of Fig. 3; Fig. 5 is a plan view from the top of the post and beam 25 support block of Fig. 1; and Fig. 6 is a plan view from beneath the post and beam support block of Fig. 1. The post and beam support block 10 illustrated in Figs. 1 to 6 includes a platform 11 having a generally a cruciate 30 perimeter in plan view and four to six rows of button-like 5 projections shown typically at 12 in a regularly spaced square pattern along the upper face of the platform. The platform is surrounded by a side wall which has an upper wall portion 13 and a lower wall portion 14, the upper 5 wall portion projecting upwardly from the perimeter of the platform and the lower wall portion projecting downwardly from the perimeter of the platform. A base wall 15 extends outwardly from the lower edges of the bottom wall portion and has an octagonal outer perimeter 16 with rounded corners. LO The upper wall portion has four sections each of which is contiguous, the four sections being separate from one another, but being the same or a mirror image of one another. Each section has a forward channel wall 18, and transverse channel wall 17 at right angles to the forward channel wall, but not L5 directly intersecting therewith. Interposed between the forward channel wall and transverse channel wall are two large socket walls 19 at 45' to each of the forward and transverse channel walls. Interposed between the large socket walls are two small socket walls 20 intersecting at right angles to one 20 another at 45' to the large socket walls at right angles to, but spaced from, the forward and transverse channel walls. Thus, taken from one end to the other, each section of the upper wall portion has a forward channel wall intersecting with a large socket wall intersecting with a small socket wall 25 which intersects with the other small socket wall which in turn intersects with the other large socket wall which in turn intersects with the transverse channel wall. The sections of the upper wall portion are arranged so that the forward channel walls are opposite one another in one 30 direction and are in substantial coplanar alignment with one another in the other direction, and being at right angles there to, and similarly, the transverse channel walls are 6 opposite one another and aligned with one another, the spacing of the forward channel walls from one another being greater than the spacing between the transverse channel walls. It will be seen that such an arrangement provides for a larger beam to 5 be supported by the platform between the forward channel walls and a smaller beam to be supported by the platform between the transverse channel walls. The sections are also arranged so that the large socket walls are opposite one another and spaced at a greater distance than the corresponding distance LO between the small socket walls. The lower wall portion follows the course of the upper wall portion to the extent of each section, but includes a sloping portion 21 and an upright portion 22, the sloping portion sloping at about 45' to the upright portion and the L5 upright portion being substantially at right angles to the base wall. Unlike the non-contiguous upper wall portion, the lower wall portion is substantially contiguous, having a slanting wall 22 interposed between the sections of the lower wall portion which correspond with the sections of the upper 20 wall portion. The slanting wall is at about 45' to the plane of the platform the base wall, but has a lower rim wall 23 connecting the lower edge of the slanting wall to the base wall. A slit 24 is provided substantially centrally and 25 penetrating through each of the slanting walls, the slit being substantially in alignment the axis of each of the channels and being sized to permit transport of humid air from beneath the platform, but prevent movement of the vermin, such as termites, through the slit. 30 A plurality of buttress walls radiate from the lower wall portion, projecting upwardly from the base wall. Four main buttress walls 25 extend outwardly at 450 from the lower wall 7 portion just below the intersection of the two small socket walls of the upper wall portion and terminate at a pinning formation 26 which has a passage therethrough at about right angles to the base wall and penetrating through the base wall. 5 The pending formation is spaced slightly inward from the outer perimeter of the base wall. Eight intermediate buttress walls 27 radiate from the lower wall portion at an intermediate angle between the 45' angle of the main buttress wall and the portion of the wall LO portion which corresponds with either the forward channel wall of the transverse channel wall of the upper wall portion, extending upward from the base wall and terminating inward from the outer periphery of the base wall. The base wall has an underside 30 as illustrated in fig. L5 6. The underside has a peripheral rib 31 and eight beam support walls 32 extending beneath the platform in substantial vertical alignment with the upper wall portions, but extending radiantly inward to terminate patent in a circular wall 37 stop two spaced apart radially spaced apart circular ribs 33 20 extend circumferentially around the lower face, intersecting with twelve buttress ribs 38 which correspond to the main and intermediate buttresses which extend from the other side of the base wall. The arcuate walls project downward from the underside of 25 the platform between the inner faces of the lower wall portion, and having a lower edge spaced upwardly from the level of the base wall as indicated at 39 in Fig. 4. Four lower buttress extension walls 35 extend radiantly inward from the inside corners of the lower wall portion which correspond 30 with the small socket walls of the upper wall portion to intersect with the circular wall. An outer circular wall 36 projects downward from the underside of the platform 8 intersecting with the lower buttress extension walls and the beam support walls. The bottom edges of the circular wall, at a circular wall, lower buttress extension walls and being support walls are substantially coplanar with the lower faces 5 of the peripheral, circular and buttress ribs. In use, the post and beam support block according to the invention may be used to support structure, such as a deck or veranda, above and on grade foundation. The toast and beam support block is particularly suited to supporting decks, more LO particularly for temporary and semipermanent structures. Although the ground or foundation upon which the structure is to be supported would preferably be flat or gently sloping, so long as the locations where the support block the of the invention are to be placed are substantially even, the ground L5 in between may be quite rough, subject only to limitation of height of mounds or such like extending above the level of the ground that supports are placed to that of the support beams supported on the support block. Support beams and support posts of different sizes can be 20 supported on the support block according to the invention. Normally, either a support post or a support beam is supported by the support block, but in some situations, a support beam may abut a support post, for example, one support beam on each side of the support post, or at right angles to one another 25 abutting support post supported by the support block of the invention. If required, the support block according to the invention may be pegged or pinned to the ground to prevent lateral movement by inserting a peg or pin through one or more of the pinning apertures and into the ground. 30 The support block according to the invention is preferably formed from a rigid engineering plastics material such as a glass or mineral reinforced polymer. The load 9 supported by the platform is transferred down and spread across the base wall through the lower wall portion and buttress walls. Although the invention has been described with reference 5 to a specific example, it will be appreciated by persons skilled in the art that the invention may be embodied in other forms within the broad scope and ambit of the invention as herein set forth and defined by the following claim.