BACKGROUND Wheelbarrows are a favoured means of transporting loads of materials from a stockpile or source of supply, to a point of use. Particularly on building sites such materials will include bricks, mortar and concrete. .Frequently the load needs to be wheeled along a sloping plank to take the load from one level to another. At other times, a load may need to be wheeled along a plank spanning some intervening space, for example across spaced apart floor beams, or over soggy or uneven terrain. A fully laden wheelbarrow requires considerable strength to control its equilibrium, particularly when the load is to be pushed up or wheeled down an incline. A further problem of negotiating an upwardly sloping incline is that a user may lose sufficient momentum or the user's feet may lose traction and the barrow start to roll back making recovery very difficult. If equilibrium is lost, the load rapidly becomes uncontrollable, tipping the barrow sideways. At the least a load may then be inconveniently lost but moreover, a user's legs may be trapped between the handle shafts of the barrow and he or she thrown over with potentially serious consequences. It is an object of the present invention to overcome or at least ameliorate some of the above disadvantages. BRIEF DESCRIPTION OF IMVEMTION Accordingly, in a first broad form of thθ invention there is provided a safety device for a builder's plank; said safety device comprising a pair of parallel rails attached symmetrically on either side of said plank; said rails spaced apart so as to underlie the stand legs of a wheelbarrow.

Preferably, said rails are provided with vertical flanges adapted to loosely constrain said stand legs therebetween.

Preferably, said rails are positioned relative said plank so as to restrict lateral tilting of a loaded wheelbarrow when said wheelbarrow is wheeled by a user along said plank.

Preferably, said rails are substantially coextensive in length with said plank.

Preferably, each of said rails comprises a length of metal angle section. Preferably, each said length of metal angle section is arranged with a first flange substantially parallel to an upper surface of said plank and a second flange substantially vertical to said surface; , each said second flange disposed at opposing outer edges of each said first flange.

Preferably, said rails are attached to said plank by a plurality of brackets.

Preferably, each of said brackets comprises a vertical element with an upper horizontal element extending in a first direction from an upper end of said vertical element and a lower horizontal element extending in a second opposite direction from a lower end of said vertical element.

Preferably, said upper horizontal element of said bracket is attached to an underside of said first flange.

Preferably, each of said plurality of brackets is attached to said plank by a hinge.

Preferably, a first leaf of said hinge is attached to a side of said plank and a second leaf of said hinge is attached to said lower horizontal element; said second leaf projecting outwardly from said side of said plank. Preferably, said second leaf of said hinge is attached to an upper surface of said lower horizontal element at an intermediate position along the length of said element such that a portion of said element projects inwardly past said side of said plank; the arrangement being such as to position said portion flush with an underside of said plank when said rails are in an in-use position; said portion providing σantilevered support for said rails.

Preferably, said hinges permit rotation of said rails between a first in-use position wherein said first flanges of said rails are in said parallel position and a second rotated position wherein said rails are positioned adjacent one to another above said plank; said rotated position compacting said plank and said rails for transport.

Preferably, each said rail is provided with a plurality of stop elements arranged at intervals along said length of said rail and transversely across upper surfaces of said first flange; said stops adapted to arrest said stand legs of a wheelbarrow when said plank is at an Incline to horizontal.

Preferably, in-fill elements are provided so as to cover gaps between outer upper edges of said plank and inner edges of said first flanges of said rails; said in- fill elements adapted to prevent feet of a user of said wheelbarrow from sliding through said gaps.

In a further broad form of the invention, there is provided a safety plank for securely wheeling a wheelbarrow therealong; said safety plank comprising an elongate extruded metal member and a pair of constraining rails; said rails coextensive and parallel with said member and arranged symmetrically one on each side of said member; said rails further arranged to prevent lateral tilting and sideways deviation of said wheelbarrow.

In yet a further broad form of the invention there is provided a method for preventing a wheelbarrow from undue lateral tilting and sideways deviation while said wheelbarrow is wheeled along a builder's plank; said method including the steps of:

(a) providing a pair of rails adapted for attachment to said plank,

(b) seccuring one rail of said pair of rails on each side of said builder's plank; said rails arranged so as to underlie the stand legs of said wheelbarrow thereby restricting lateral tilting of said wheelbarrow, {c) providing said rails with outer vertical flanges spaced apart so as to restrict sideways movement of said stand legs-

In still a further broad form of the invention there is provided a method for arresting a wheelbarrow on an inclined builder' s plank when said wheelbarrow is in danger of uncontrolled motion along aaid plank; said method including the steps of:

(a) providing a pair of rails adapted for attachment to said plank,

(b) securing one rail of said pair of rails on each side of said builder's plank; said rails arranged so as to underlie the stand legs of said wheelbarrow,

(c) providing stops at intervals along said rails; said stops adapted to engage with the stand legs of said wheelbarrow when said stand legs are lowered to the surface of said rails.

In still a further broad form of the invention there is provided a builder's plank assembly; said assembly comprising an extruded elongate plank member and a pair of guide rails attached one to each side of said plank member; each of said pair of guide rails comprising a length of angle section substantially coextensive in length with said plank member and arranged such that a first flange of each said angle section underlies one of the stand legs of a wheelbarrow so as to restrict lateral tilting of said wheelbarrow; and wherein opposing second flanges of said angle section restrict sideways movement of said stand legs.

BRIEF DESCRIPTION OF DRAWINGS Embodiments of the present invention will now be described with reference to the accompanying drawings wherein: Figure 1 is a perspective view of a wheelbarrow at rest supported on a preferred embodiment of a plank safety device according to the invention, Figure 2 is an enlarged perspective view of an end portion of the plank safety device of Fig. 1, Figure 3 is a perspective view of the plank safety device of Fig. 1 with side rails in a rotated state for transport. Figure 4 is a front elevation view of a wheel barrow in position on the plank safety device of Figs. 1 and 2. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS With reference to Fig. 1, a plank safety device 10 comprises two parallel rails formed of lengths of angle section 12 and 14 disposed symmetrically on either side of, and a little above, a conventional builder's plank 16. The length of angle sections 12 and 14 is substantially equal to the length of plank 16. The height of the horizontal flanges 21 and 23 of the angle sections 12 and 14 above the upper surface of builder's plank 16 is such as to be somewhat less than the height above ground level of the lower ends of a wheelbarrow's stand legs when in motion. The term "stand legs" in this specification signifies the rear supporting frame, usually formed as a pair of braced legs, depending from the body or chassis rails of a wheelbarrow, which support the barrow when at rest. Each of angle sections 12 and 14 is supported on a plurality of brackets 18 provided at intervals along the length of plank 16. The spacing between the inside surfaces of vertical flanges 13 and 15 of angle sections 12 and 14 is such as to allow the stand legs 20 of a typical full- sized builder' s wheelbarrow 22 to fit loosely between them. The width of the horizontal flanges 21 and 23 of each of angle sections 12 and 14, is sufficient to support the stand legs 20 of the wheelbarrow 22, while the height of vertical flanges 13 and 15 is such as to restrict sideways movement of the lower ends of legs 20 when the wheelbarrow 22 is wheeled along the plank 16 by a person of average height. As may be seen more clearly in Fig. 2, builder's plank 16 is preferably, though not necessarily, a length of extruded metal, forming a series of conjoined elongate cells. In this preferred embodiment, brackets 18 are of a generally "Z" configuration, with a vertical member and upper and lower horizontal members projecting in opposing directions from respective upper and lower ends of the vertical member. The upper horizontal member 24 is fastened to the underside of the horizontal flanges 21 and 23 of the angle sections 12 and 14, and the lower horizontal member 26 is attached to the first leaf 25 of a hinge 28, the second leaf 27 of which is in turn fastened to the side of plank 16. It can be seen in Fig. 2 (and more clearly in Fig- 3) that the lower members 26 of brackets 18 are longer than the upper members 24 and extend inwardly past hinges 28 so as to underlie the underside of plank 16 when the angle sections 12 and 14 are in the operational position of Figs. 1 and 2. By this means, angle sections 12 and 14 are supported in a cantilevered manner to bear the load transmitted to flanges 21 and 23 by the stand legs 20 of a loaded wheelbarrow. The arrangement of attachment of angle sections 12 and 14 to plank 16 by means of hinges 28 allows the angle sections to be rotated inwardly as shown in Fig. 3, to make the safety plank device more compact for ease of transport. Again with reference to Fig. 2, each of angle sections 12 and 14 may be provided with a plurality of stops 30 (only one of which is visible in Fig. 2) . Stops 30 may take the form of angle brackets fastened to the horizontal flanges 21 and 23 of angle sections 12 and 14. Stops 30 provide emergency stop facilities to arrest a wheelbarrow which is becoming uncontrollable on a sloping plank, by engaging with the stand legs of the barrow. These stops also allow a wheelbarrow to be Sparked" if required on a sloping plank. In use, a wheelbarrow is wheeled along a plank fitted with the safety device of the present invention, with the lower ends of its stand legs just clear of the horizontal flanges 21 and 23. Any tendency for the wheelbarrow to tip sideways is then quickly arrested ensuring that the transit along the plank may be made in safety. Deviation of the wheelbarrow from the line of the plank is prevented by the restriction of sideways movement of the stand legs by the vertical flanges 13 and 15. Should the plank be inclined and the user of the wheelbarrow lose control so that the barrow is in danger of running back over the user or away from him or her, the stops provided at intervals along the length of the angle sections will bring the barrow safely to a halt. In a further preferred embodiment of the invention, the brackets supporting the angle sections are not fastened to the builder's plank by hinges but are directly attached. In this embodiment the bracket may still be of a WZ" form as previously described, but now the lower horizontal section of the bracket is bolted or otherwise attached directly to the underside of the plank. In still a further preferred form of the invention, the angle section supporting brackets are provided with adjustments to allow the separation between the vertical flanges of the angle sections to be varied to suit the stand leg separation of a particular preferred wheelbarrow. For this purpose, slots may be provided in the upper or lower horizontal members (or in both these members) . With reference to Fig. 4, in at least one further form of the invention, the gaps between the outer edges of plank 16 and the internal edges of the horizontal flanges 21 and 23 of rails 12 and 14 are provided with safety in-fills 30 to prevent the feet of a user of the wheelbarrow 22 from inadvertently slipping between the plank and rails. The above describes only some embodiments of the present invention and modifications, obvious to those skilled in the art, can be made thereto without departing from the scope and spirit of the present invention. Thus it will be understood for example, that the angled sections described above may be attached to a timber plank with suitable brackets and that channel sections could be employed in place of angle sections. In particular forms the plank described above can be utilised, in effect, as a ramp for the purpose of bridging a gap between two locations which are either at the same level or at differing levels. Accordingly all references to "plank" in this specification can also be read as wramp" when the plank above described is used in this manner and for this purpose.