TILING ACCESSORY Technical Field This invention relates to a tiling accessory and in particularly to an accessory which enables persons to walk over newly laid tiles before the application of grout to the grout joints or grooves between tiles. Background Art During the laying of floor tiles in a specified area such as a room, a floor tiler normally commences tiling from the centre of the room or one edge and lays tiles on the floor outwardly towards the opposite edges or walls of the room. As the laid tiles reach the proximity of the walls of the room or to the edges up to which the tiles are to be laid, the tiler is often required to cut the tiles to complex shapes to match the profile of the wall or other edge. In most cases, the laying of the uncut tiles can be usually accomplished relatively rapidly. Usually however the tiler after laying those tiles, leaves the tile adhesive to set so that on the next day the tiler can walk on the laid tiles and then finish the tiling task by completing the cutting and laying tiles at the edge of the room or other area. The tiler is thus involved in the tiling task for at least one extra day and this therefore leads to increased costs. Further it limits opportunities for the tiler to take on jobs on other sites. Tiling of floor areas also limits access to or through the area or room in which the tiling task is being carried out. Depending upon where the area being tiled is located, access can be limited to other parts of a building. Thus, for example, in the case of construction of new buildings, other tradesmen may be unable to carry out work in these parts of the building until the tiling has finished. As a result of these time delays, overall costs of the building are increased. Summary of the Invention The present invention aims to overcome or alleviate one or more of the above disadvantages by providing a tiling accessory which enables persons to walk over newly laid tiles without disturbing the tiles. The present invention also aims to provide a method for providing access over a floor or base surface carrying newly laid tiles. Other objects and advantages of the invention will become apparent from the following description. The present invention thus provides a tiling accessory for providing a support over tiles laid over a floor or base surface and having grout lines or grooves therebetween, said accessory comprising a platform, and a plurality of legs depending from the underside of said platform, said legs being of a width to enable them to be positioned in said grout lines or grooves to seat on said floor or base surface, said legs being further of a height such that said platform in use is supported through said legs on said floor or base surface and is positioned above said tiles. The platform preferably is of a rectangular configuration and most preferably of a substantially square configuration in plan view and respective legs are positioned adjacent the respective corners of the platform. The corners of the platform may be truncated. Preferably the platform has a substantially planar upper support surface. The sides of the platform between adjacent corners are may be concave in plan view. Preferably a first pair of legs are arranged adjacent to the respective corners of one pair of opposite corners of the platform and a second pair of legs are arranged adjacent to the respective corners of the other pair of opposite corners of the platform. Preferably the legs of the first pair are aligned along the diagonal between the one pair of corners and the legs of the second pair are aligned along the diagonal between the other pair of corners. Preferably the legs comprise flange-like feet of fins which lie in planes extending substantially normal to the platform. The flange-like feet or fins suitably extend along the respective diagonals. The feet or fins are less than the width of the grout lines or grooves so that they may locate freely therein and seat on the floor or base surface. Furthermore the feet or fins are of such a height that the platform is located in use above the surface of the tiles. The accessory also may include one or more intermediate flange-like feet or fins in a central portion of the platform to provide support to the platform in this region. The one or more flange-like feet or fins suitably extend along one diagonal and are aligned with the opposite corner flange-like feet or fins. Preferable the intermediate feet or fins are of a height less than the height of the corner feet so that the intermediate feet or fins do not contact the surface on which the tiles are being laid until a predetermined force is applied to the platform such as by a person standing on the platform. In use the legs adjacent one pair of opposite corners are located along one grout line or groove defined between adjacent tiles and the legs adjacent the other pair of opposite corners are located along an orthogonally extending grout line or groove defined between the tiles. Thus the accessory may be positioned centrally over the junction of four adjoining tiles defined by intersecting grout lines or grooves. The floor or base surface upon which the tiles are laid may be the surface of a concrete slab, the surface of a timber floor or any other indoor or outdoor base surface. The present invention in a further aspect provides a method for providing access over an area of tiles laid on a floor or base surface, said tiles having orthogonally extending grout lines or grooves therebetween, said method including the steps of providing a platform having a plurality of legs depending from the underside thereof, and positioning said platform relative to said tiles such that opposite pairs of said legs of said platform locate in the respective orthogonally extending grout lines or grooves to seat on said floor or base surface and support said platform above said tiles. Preferably the method includes the step of providing a plurality of platforms, each having a plurality of legs depending from the underside thereof for location in the grout lines or grooves, and positioning the platforms at spaced apart positions over the area of tiles to provide a walkway over the tiles. Brief Description of the Drawings In order that the invention may be more readily understood and put into practical effect, reference will now be made to the accompanying drawings which illustrate a preferred embodiment of the invention and wherein :- Fig. 1 is a view from above of a tiling accessory according to the invention; Fig. 2 is an underside view of the accessory of Fig. 1 Fig. 3 illustrates in enlarged view one support leg of the tiling accessory; Fig. 4 is a view from above of the tiling accessory applied over laid tiles; Fig. 5 is a cut away perspective view illustrating the manner in which a support leg of the accessory is received in a grout line or groove between tiles and seats on a floor or base surface; Fig. 6 illustrates the manner in which a plurality of tiling accessories are used to form a walkway over tiles; Fig. 7 is a sectional view of a side edge of a modified tiling accessory; Fig. 8 is a plan view of an alternative tiling accessory according to the invention; and Fig. 9 is a side view of the tiling accessory of Fig. 8. Detailed Description of the Preferred Embodiment Referring to the drawings and firstly to Figs. 1 and 2, there is illustrated a tiling accessory 10 according to an embodiment of the present invention comprising a rectangular platform 11 which in the illustrated embodiment is of substantially square configuration in plan view but which has its four corners truncated as at 12. Typically the platform 11 is formed of timber or plastics and is sufficiently rigid to support the weight of a person standing or walking thereon. The upper surface 13 of the platform 11 further is substantially planar to provide a flat support surface. On the underside of the accessory 10, a plurality of legs 14, in this case four legs 14, are provided each being located adjacent a truncated corner 12 of the platform 11. In addition, pairs of legs 14 in opposite corners 12 are aligned along the respective diagonals 15 extending between opposite truncated corners of the platform 11. The legs 14 as more clearly shown in Fig. 3 are in this embodiment of a T-shaped cross section having a base flange 16 which may be attached to the underside 17 of the platform 11 such as by screws driven through apertures 18 in the base flange 16 and which are positioned to extend inwardly from the corners 12. The legs 14 additionally include a centre flange or fin 19 extending at right angles from the base flange 16 such that in use the centre flange 19 lies in a plane extending normal to the underside 17 and top surface 13 of the platform 11. Opposite outer corners of the centre flanges 19 are suitably rounded as illustrated for safety. Additionally as referred to above the centre flanges 19 of the legs 14 in the opposite corners 12 are aligned with and extend along the diagonals 15 between those corners 12. The centre flanges 19 are of a width less than the width of the normal grout lines or grooves between adjacent tiles and are further of a height greater than the thickness of the tiles plus tile adhesive with which the accessory 10 is to be used. In use and as shown in Figs. 4 and 5, the accessory 10 is used typically during the laying of tiles 20 using a tile adhesive on a floor surface 21, the tiles 20 being square in plan view and defining therebetween a plurality of orthogonally extending sets of grout lines or grooves 22 and 23 for receiving grout. Where a person desires to walk or be supported over the floor surface 21 without disturbing the newly laid tiles 20, the accessory 10 is positioned such that the centre flanges 19 of the aligned legs 14 between one pair of opposite corners 12 are received in the grout lines or grooves 22 so as to seat on the underlying floor surface 21 and similarly the centre flanges 19 of the aligned legs 14 between the other pair of opposite corners 12 are received in the grout lines or grooves 23 to again seat on the underlying floor surface 21. A person may then stand or squat on the upper surface 13 of the accessory 10 and carry out further tiling operations such as at the opposite sides of the laid tiles 20 for example where tiles are required to be cut to meet an edge. As the legs 14 are provided adjacent the corners 12, a workman such as a tiler can move around the upper surface 13 of the accessory 10 and be fully supported without risking tipping of the accessory 10. If additional support is required centrally of the accessory 10, a further leg 14' of the same configuration as the leg 14 of Fig. 3 may, as shown in dotted outline in Fig. 2, be fixed to the underside 17 of the platform 11 with the centre flange 19' thereof aligned with one of the diagonals 15. The accessory 10 may also be simply used to provide access to a particular area without the necessity of walking on the tiles 20 which would not be possible where the tiles 20 are freshly laid without disturbing the tiles 20. Further a plurality of accessories 10 may be used to define a walkway 24 along a tiled area 25 as shown in Fig. 6. This arrangement may provide access along for example a hallway or in any area where tiles have been freshly laid. Of course after the adhesive of the tiles has set, grout may be applied to the grout lines or grooves 22 and 23. For this procedure, the accessories 10 may be left in place to provide convenient access to the grout lines or grooves 22 and 23 and then removed one by one where access to the portions of the grout lines or grooves 22 and 23 beneath the platforms 11 for grouting is required. Typically the platform 11 is formed of timber and the legs 14 form of metal such as aluminium. The platform and the legs however may be formed of other materials such as a plastics material. For example the platform and legs may be moulded integrally of plastics. The platform further whilst shown to have truncated corners may have non-truncated corners. Whilst the legs 14 in the preferred embodiment are of a T-shaped configuration, they may be of other configurations provided that they are sufficiently strong to support the weight of a person standing on the accessory 10. Typically the platform 11 is approximately 350 mm square and the feet of the legs extend approximately 20 mm outwardly from the underside 17 of the platform. These dimensions however may vary where for example the platform 11 is required to be elevated further above the tiles 20. The feet typically have a thickness of 1.5 to 1.7 mm for location in the grout lines or grooves 22 and 23 and are approximately 30mm in length. These dimensions however may also vary. The legs 14 may also be in configurations other than that illustrated in the embodiments. Whilst the platform 11 is most preferably square or substantially square in plan view, it may be of other shapes such as octagonal or circular with the legs 14 being on the underside of the platform and arranged along mutually perpendicular lines so that they may locate in orthogonally extending grout lines or grooves. The platform 11 may have three or more legs but most preferably has at least four legs 14 to provide a stable support surface. To facilitate lifting and manipulation of the accessories 10, the platforms 11 may be provided with a concavity 26 in their outer side edge or edges as shown in cross section in Fig. 7. The concavity 26 serves as a convenient recess to receive the fingers of the hands of the user to enable the accessories 10 to be more easily lifted or moved. Whilst the accessory 10 is illustrated in the embodiment as being used with relatively large square floor tiles, the accessory 10 of the invention may be used with tiles of any size which when laid define orthogonally extending sets of grout lines or grooves which can receive the legs 14 of the accessories. Referring now to Figs. 8 and 9, there is illustrated an alternative tiling accessory 27 according to the present invention including a platform 28 defined by a thin planar member 29 stiffened by a series of ribs 30 on its underside and a peripheral skirt 31 , each being integrally formed with the member 29. The accessory 27 is formed with four flanges or fins 32 adjacent each corner of the platform 28, the fins 32 lying in diagonal planes extending between the corners of the platform 28 at right angles to the plane of the planar member 29. The fins 32 are of a width to enable them to locate in the grout line between adjacent tiles in a similar manner to the flanges 19 of the embodiment of Figs. 1 to 6. The accessory 27 additionally includes two further fins 33 which also lie in one of the diagonal planes extending between one pair of opposite corners, the fins 33 being positioned symmetrically on opposite sides of the center of the platform 28. The fins 33 are of a similar thickness to the fins 32 to allow their location in a grout line or groove however they are of a height less than that of the fins 32 so that they do not provide support to the platform 28 until a weight of a person is placed on the platform 28 and deflects the platform 29 downwardly. This ensures that the accessory 27 will sit stably on the underlying surface being supported by the four corner fins 32 with supplemental support being provided by the central fins 33 when a person steps onto the platform 28. The platform 28 as illustrates is of a substantially square configuration however the sides 34 thereof are of a concave configuration between adjacent corners. This will direct persons using the accessories to stand on a central part of the platform 28 to ensure stable support. The accessory 27 of Figs. 8 and 9 is used in the same manner as the accessory 10 and as described with reference to Figs. 5 to 7. The accessory 27 is preferably moulded of plastics such as an injection moulding process with the parts thereof formed integrally. The terms "comprising" or "comprises" or derivatives thereof as used throughout the specification and claims are taken to specify the presence of the stated features, integers and components referred to but not preclude the presence or addition of one or more other feature/s, integer/s, component/s or group thereof Whilst the above has been given by way of illustrative embodiment of the invention, all such variations and modifications thereto as would be apparent to persons skilled in the art are deemed to fall within the broad scope and ambit of the invention as herein define in the appended claims.