The present invention relates to a material for providing a surface having the appearance of brickwork. The invention also extends to a method for producing the material and a method of providing a surface using the material. In some circumstances, it is desirable to give the impression that a wall is built from bricks and mortar, when in fact it is not. For example, it may be desirable to change the look of a pre-existing wall. Building walls from bricks and mortar is also expensive and time-consuming. Therefore it may be desirable to build a wall using a less expensive method and then provide a surface decoration that gives the impression of a brick wall. It is known to decorate walls with brick slips or tiles that are manufactured from fired clay. The tiles are adhered to a wall and the joints between the tiles are pointed to give the aesthetic impression of a brick wall. However, such tiles are relatively expensive and are also time-consuming to apply to the wall. It would therefore be desirable to provide an alternative material that could be used to decorate a wall with a brick effect. Viewed from a first aspect, the present invention provides a method of forming a brick-effect material comprising: providing a substrate; forming at least one brick-effect region on the substrate from a mixture comprising brick particles and adhesive; and allowing the mixture to harden. Thus, the present invention provides a relatively inexpensive way to produce a material with the appearance of brick. Viewed from a second aspect therefore, the present invention provides a decorative material comprising a substrate and at least one brick-effect region on the substrate, wherein the brick-effect region is formed from a mixture comprising brick particles and adhesive. Thus, the present invention provides a relatively- inexpensive material with the appearance of brick. In general, the brick-effect region is substantially rectangular in order to give the appearance of the visible surface of a standard brick. Thus, the brick-effect region typically has a length and height corresponding to that of a standard brick, i.e. 225 mm by 75 mm. The thickness of the brick-effect region is preferably less than 25 mm, and in the preferred embodiment is approximately 12 mm. It is advantageous to make the brick-effect region as thin as possible while maintaining the desired appearance in order to minimise the amount of material required to manufacture the material and to produce a lightweight panel. Preferably, the material comprises a plurality of brick-effect regions each having the appearance of a discrete brick. The brick-effect regions may be arranged in a brickwork pattern on the substrate. Preferably, the brick-effect regions are spaced and the spaces between the brick-effect regions are pointed. In this way, a panel may be formed which can be mounted on a surface, such as a wall, and instantly provides the surface with the appearance of brickwork. Such panels are relatively inexpensive and simple to produce and are quick and easy to fix to existing structures or frames. In this way, a brick-effect surface can be installed in a building in a greatly reduced time and at greatly reduced cost. Viewed from a further aspect therefore, the invention provides a brick-effect panel formed according to the above method or from the above material. Viewed from a yet further aspect, therefore, the invention provides a method of providing a surface having the appearance of brickwork, the method comprising fixing a plurality of the above panels to a support. The support may be an existing wall or floor or may be a support structure, such as a timber framework. Thus, the panels of the invention may be used as building materials mounted to a framework to create walls with the appearance of brickwork. The arrangement of brick-effect regions may include at least one gap devoid of a brick-effect region proximate the periphery of the panel. In particular, the gap may be dimensioned to be approximately half the length of the brick-effect regions. Thus, the brick effect regions which would extend beyond the edge of the panel are not formed on the substrate. If, for example, the brick-effect regions are arranged in a traditional brickwork pattern with each row of bricks offset from the neighbouring rows by one half brick length, a gap corresponding to a half-brick length is defined at the end of alternate rows on the panel. With such a construction, the boundaries between adjacent panels are obscured by filling the gaps with individual brick- effect tiles after the panels have been fixed to the surface to be decorated. Conveniently, fixing points, such a screw holes, for the panels -may be provided in the gaps, and these will be covered by the individual brick effect tiles in the finished surface. Viewed from a yet further aspect, therefore, the invention provides a method of providing a surface having the appearance of brickwork, the method comprising: fixing a plurality of the panels to a support such that the gaps on adjacent panels form a space for at least one additional brick-effect region; and fixing, for example with adhesive, at least one additional brick-effect region in the space. The method may further comprise pointing around the additional brick-effect region. Alternatively or additionally, the periphery of the panel may be substantially coterminous with the edge of the brickwork pattern of the brick-effect regions. In this way, the panels may interlock to form a continuous brickwork pattern. The interface between adjacent panels may be pointed when the panels have been mounted to the surface to be decorated. The substrate may be flexible. Preferably, however, the substrate is substantially rigid. For example, the substrate may be a sheet or board of any suitable material. For reasons of cost and availability, a wooden board is preferred and in the particular embodiments described, the substrate is a plywood board. If desired, the substrate may be formed of a fireproof material. The substrate may have a textured or roughened surface to ensure satisfactory adhesion of the brick-effect regions. For example, the substrate may be textured with a saw. The brick particles may be of any suitable size. For example, the brick particles may have a maximum diameter of 5 mm or less, preferably 3 mm or less. Crushed brick, brick powder or any other particulate brick material may be used. The brick particles need not be formed from brick clay, but-may be formed from another material, such as crushed limestone, coloured as necessary to provide a brick effect in the resultant material. Any suitable adhesive may be used in the mixture. For reasons of cost and availability, a particularly preferred adhesive is polyvinyl acetate (PVA) . The presently preferred adhesive is Evo-Stik 9517 multi- purpose PVA. If desired, a cross-linker may be added. The mixture may comprise additional components, for example water. The preferred mixture comprises one part adhesive, one part water and sufficient crushed brick to make a flowing mixture. In one preferred embodiment, the mixture further comprises glass fibres. In forming the material, the mixture may be applied to the substrate or the substrate may be applied to the mixture. An adhesive layer may be provided between the substrate and the mixture. The adhesive in the adhesive layer may be the same adhesive as that in the mixture or a different adhesive. Conveniently, the same adhesive is used. The mixture may be formed on the substrate in a continuous layer and the discrete brick-effect regions may be defined by removing or displacing portions of the mixture, for example by stamping a brickwork pattern onto the layer of mixture. In the presently preferred method, however, the brick-effect region is defined by a mould for the mixture, which preferably defines a plurality of brick- effect regions. The mould may be positioned on the substrate (and adhesive layer, if required) and the mixture may be poured into the mould. Alternatively, the components of the mixture may be mixed in the mould. The mould may be vibrated, or otherwise mechanically agitated, with the mixture in the mould, to allow the mixture to slump and fill the voids in the mould. Compression forces may further be applied to the upper surface of the mould to further compress the mixture. Conveniently, the mould may be formed from a sheet of material with cut-outs to define the brick-effect regions. A suitable material is medium density fibreboard (MDF), fibre glass, rubber or other elastomer. The edges of the mould which define the brick-effect regions may be tapered to allow easy separation of the mould from the material. The mixture may be allowed to harden naturally. However, for rapid manufacture, the mixture is preferably hardened by heating or a similar hardening process. For example, heated air may be applied to the mixture on the substrate, for example by blowing the heated air at the surface of the mixture. The mould, if used, may be removed when the mixture has hardened. After the mould has been removed, the mixture and substrate are preferably baked in an oven at about 25O0C for about 20 minutes. The joints between the brick-effect tiles are preferably pointed after the mixture has hardened. Any suitable pointing mixture may be used, for example three parts fine block paving sand, one part mortar cement, one part water and one part PVA. According to an invention disclosed herein there is provided a decorative panel comprising a plurality of brick-effect regions mounted on a substrate to give the appearance of brickwork. The brick-effect regions may be mutually spaced and pointed in the spaces between adjacent brick-effect regions. An embodiment of the invention will now be described by way of example only and with reference to the accompanying drawings which are not drawn to scale and in which: Figure 1 illustrates the formation of a brick- effect panel according to the invention; Figure 2 is a perspective view of a panel made as shown in Figure 1; and Figure 3 is a view of a wall formed from the panels of Figure 2. With reference to Figure 1, an 8 ft x 4 ft x 1/2 in (2.4 m x 1.2 m x 12 mm) plywood board 1 is laid on a flat table. The plywood board 1 is coated with PVA adhesive (not shown) and a cut-out mould 2 defining a brickwork pattern is positioned on the board 1. The depth of the mould 2 is typically 12 mm, which represents the desired thickness of the brick pattern on the panel. To fill the mould 2, a mixture 3 of crushed brick, PVA adhesive and water is made up. Brick particles having a diameter from 3 mm down to small dust particles can be used. The components of the mixture are typically combined with sufficient crushed brick and one part adhesive to one part water in order to form a flowing mixture 3. In one possible embodiment, the mixture also includes glass fibres. The fibres are typically 6 mm long and are mixed into the mixture at a ratio of approximately 0.9 kg of glass fibres per m3 of crushed brick. The mixture 3 is then poured into each compartment of the mould 2 by travelling. The table upon which the board 1 is placed has a vibratory action, which allows the mixture 3 to slump and fill the voids of the mould 2. If required, the mixture can be further compressed within the mould using a roller applied to the upper surface of the mould. Alteratively, the compression could be piston driven or a roller press could be used. Once all the compartments of the mould 2 are filled, heat is applied to the surface of the mixture 3 by blowing warm air over it to set the mixture 3. Approximately 15 minutes after heat application, the mould 2 is released leaving a panel 4 with the appearance of a brick wall, as shown in Figure 2. To dry the mixture, after the mould has been removed, the panel is placed in an oven for about 20 minutes at 2500C. As shown in Figure 2, the panel 4 has every half brick on the sides left blank. Thus, when the panels 4 are fixed next to each other, as in the wall of Figure 3 for example, the half brick spaces 5 line up to make full brick spaces. These spaces 5 can then be filled . with a full-size brick slip 6 made from the mixture of crushed brick, water and adhesive. Once the brick-effect panel 4 has dried sufficiently, the joints 7 between adjacent bricks are pointed with a mortar paste composed of three parts fine block paving sand, one part mortar cement, one part water and one part PVA adhesive. Thus, the overall effect of the panel 4 is that of a brick wall. The panels 4 can be fixed to a framework by screwing at intervals into pre-spaced timber frame uprights 8. If half brick spaces 5 have been left along the edges of the panel 4, the panels 4 are lined up so that these half brick spaces 5 align and leave full brick spaces at every other row along the join. These spaces are filled in by adhering pre-made brick dust brick slips 6 into these voids and further pointing with the mortar paste. The panels 4 can be used for turning corners to reproduce a brick corner effect. In the case of window reveals, the overall effect does not differ from a traditional brick building except that the brick-effect panel 4 is perfect. The weight of one panel 4 is easily managed by two persons. However, if weight is an issue, the thickness of the board 1 and/or the thickness of the brick-effect layer may be reduced, which would also reduce cost. The major application of the panels 4 is in pre¬ fabricated or timber-framed buildings. The advantages over the traditional system of bricklaying are speed, a relatively lightweight structure and the cost of the finished structure. The panels 4 may also be used in other applications, such as mobile home cladding, sectional site offices, showroom display, exhibition work or DIY home improvement. Although the present invention has been described in the context of brick-effect panels, the invention could be applied to panels providing other effects. For example, it may be possible to provide a stone-effect panel using a mixture of sand and adhesive. In summary, a brick-effect panel is formed on a substrate by pouring a mixture of brick particles and adhesive into a mould and allowing the mixture to harden. The brick-effect panels are relatively inexpensive and can be used to quickly construct a wall with the appearance of brickwork.