INTERLOCKING BUILDING BRICK This invention relates to an interlocking building brick. It relates particularly to a building brick that makes it easy for the non-expert bricklayer to construct structures such as garden walls. Various models of interlocking building brick have been proposed over the years and one example is disclosed in patent application No. EP 1022973. The patent application describes a building brick which includes connectors such that a protrusion on one brick is able to be engaged in a depression in another brick. The method proposed is a dry walling system which does not use any jointing agent. This is thus very suitable for making a temporary structure such as a garden barbecue base. When a brick structure is made in this system, the brick major faces are pressed into direct contact with one another and this helps to give the resulting dry walled construction its structural strength. Since the resulting structure is made without the use of mortar, the appearance of the brick assembly will be different from the normal appearance of a brick wall which always shows a line of mortar between each pair of bricks. The resulting structures therefore do not look like a traditional brick construction and it will be apparent that they were not constructed by a professional bricklayer. The present invention was devised to provide an interlocking building brick that would have its own means to control its location and spacing relative to similar adjacent bricks. Additionally, the brick would be able to be used with a jointing agent so that the appearance of the interlocking brick structure would be very similar to that of a structure made from traditional bricks. According to the invention, there is provided an interlockable building brick having on a first major face two projections and on a second major face two depressions the axes of which depressions are coincident with the axes of the said projections, the dimensions being such that when one brick is fitted on top of another the confronting brick major surfaces will be held apart from one another by a predetermined distance. The said predetermined distance defines the minimum size of a mortar gap which is left between the bricks. The brick depressions may be circular in cross-section. The brick projections may be circular in cross-section. The brick depressions may include an internal shoulder effective to support a projection above a bottom surface of the depression. Each brick may be provided with two projections and two depressions. The volume of the two depressions on a brick may be arranged to act as a measure of the volume of mortar which will be required to form a joint between two bricks when these have been fitted together. A side of the brick carrying the depressions may additionally include a recess forming a 'frog' such that an improved supporting key for the jointing mortar will be made. The invention also includes bricks made in half-unit, one-and-a-half unit, double and multiple unit sizes. Also, bricks having only the depression feature and not a corresponding projection feature. Similarly, bricks having only the projection feature and not the depression feature are also included. By way of example, some particular embodiments of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a perspective view of the brick of the invention, Figure 2 is a view from one side, Figure 3 is a view from one end, Figures 4 and 5 are respectively the views from above and below, Figure 6 is a view in cross-section showing how the brick units are fitted together to form an interlinked structure, Figures 7 and 8 are respectively plan and perspective views of a different embodiment, Figures 9 and 10 are respectively plan and perspective views of a third embodiment, Figure 11 is a side view of the third embodiment, Figure 12 is a perspective view from below, Figure 13 is a perspective view of this embodiment in a form intended to be used as a 'base' brick of a wall construction, Figure 14 is a partial view of a wall constructed with the bricks of the third embodiment, and, Figure 15 is a cross-sectional view of a wall assembled from the top and base bricks. As shown particularly in Figure 1, the building brick 1 is of such a size and external appearance as would correspond to a conventional house brick. In the present example, the dimensions are 215 x 102.5 x 65 millimetres. An upper face of the brick 1 is provided with a shallow recess 2 which is intended to act as a 'frog' for eventually supporting a body of mortar to improve the adhesion of a brick which is to be placed on top. A base surface of the recess 2 carries two depressions 3 which are circular in cross-section and which extend about halfway through the depth of the brick. A lower face of the brick is provided with two projections 4 which are generally conical in shape. Figure 1 also shows a support shoulder 6 which is present in the depressions 3. This shoulder 6 is intended to support the end of a projection 4 when this has been inserted in the depression. The presence of the shoulder thus serves to hold the end of the projection away from the bottom surface of the depression. Figures 2 and 3 show elevation views of the brick shape taken respectively from the brick side and brick end. These views show the projections 4 extending from the lower face of the brick 1. Figure 4 is a view from above of the brick upper face. The recess 2 surrounds the two depressions 3 so that when a body of mortar is poured into the recess, this will fill the depressions 3 and an upper Hp of the recess 2 will indicate when a suitable predetermined volume of the mortar is present. Figure 5 is an underneath view of the brick lower face. The projections 4 which have been said to be conical in shape are now seen to be more in the shape of a square pyramid. This provision ensures that when a projection 4 is fitted into a depression which has been filled with a liquid mortar, most of the volume of the mortar will be pushed out of the depression. However, there will still be a proportion of the mortar lying between the side walls of the pyramid shape and the internal walls of the depression to ensure the formation of a strong joint when the mortar hardens. In a different embodiment, it would of course be possible to use a conical projection which would make a wall-to-wall contact with the conical internal shape of the depression. Such a construction would however act to squeeze out most of the mortar from between the surfaces and a strong mechanical joint would not be attained. Figure 6 is a cross-sectional view showing the brick units being fitted together to form a wall structure. It will be noticed that the brick A has a projection 4 which has been inserted into a depression 3 on the brick B. The depression 3 is provided with the internal shoulder 6 so that the projection 4 will be supported above the bottom of the depression. The two bricks A and B are thus supported a predetermined vertical distance apart from one another and this spacing has left a gap all round the bricks which would retain the usual line of mortar which is visible at the edge of bricks in traditional brickwork. Figure 6 also shows a variation of the interlocking bricks where the brick B is seen to have only the depression features and not the projections. This model of brick B would therefore be suitable for laying first on the ground surface for starting the building of a wall construction. Similarly for finishing off the top line of bricks on a wall, the uppermost row of bricks would be provided only with the projection feature and not the depression feature. Figures 7 and 8 show an alternative embodiment of building brick where the projections 4 on the brick lower surface which were of a generally square cross-section have been replaced by circular projections 7. On the brick upper surface, the depressions 3 which originally were circular have been replaced hy 'daisy'-shaped depressions 8. The 'daisy' shape is still formed with a support shoulder but this shoulder has been divided up into eight shoulder segments 9 which are located on the circumference of a circle. Between each pair of segments 9, a flow channel 11 is located. The purpose of the channels 11 is to allow a liquid mortar composition to be readily pushed out of the depression when the projection 7 of another brick is being inserted. The channels 11 thus allow the mortar to flow into the recess 2 forming the 'frog' area. They also provide some free space for the mortar to be compressed into without affecting the location of the upper brick in an assembly of the bricks. It will be noticed that in the embodiment of Figures 7 and 8, the features of the brick which assist its correct location on the lower bricks of an assembly have been moved to the depressions 8. This ensures that the location points are essentially provided in the depressions so that these points will be protected against impact damage. Since bricks are sometimes handled in a rough manner it is helpful if the location points can be protected against damage. In operation of the interlocking building brick of the invention, the geometry of the brick makes it easy for the bricklayer to measure the correct amount of mortar to form a joint Λvith other bricks. The bricklayer will apply the mortar with a trowel into the two depressions 3 and into the recess 2 of a laid brick until the mortar surface is level with the upper face of the brick. A new brick is then laid on top and pressure is applied to push the projections 4 of this brick into the depressions 3. This action serves to displace most of the mortar volume from out of the depressions and into the joint area between the bricks until the new brick has been correctly located in position. Pointing of the joint is then carried out in the usual way. The use of the bricks can significantly speed up the laying process as compared to conventional bricks and they keep waste of mortar to a minimum, keep the brick spacing more uniform and they will help to keep the brick surfaces cleaner. A wall built with the new bricks will have improved strength due to the interlocking of the bricks and the rotation feature provided by the use of circular depressions can enable walls which, are circular or non-perpendicular to be built. Figure 9 is a plan view and Figure 10 a perspective view of a third embodiment of the building brick. In this embodiment, the 'daisy' - shaped depressions 8 have been replaced by a 'four-petalled flower' pattern 12 of depressions. Between each pair of petals of the pattern 12, a height control plinth 13 is located. The position of the plinth 13 thus determines the minimum vertical spacing between the bricks when one is placed on top of another. The circular projections 7 on the bottom of the brick have been increased in volume so that they will be able to displace a large volume of mortar from a corresponding mortar filled space on a lower brick. The ability to displace this quantity of mortar is arranged so that the mortar joint between two assembled layers of bricks will be fully filled with the displaced volume of mortar. This ensures that the builder will be able to finish the mortar joint using just a pointing tool and additional mortar to fill the joint space will in general not be needed. Figure 11 is a side view of the brick of the third embodiment and Figure 12 is a perspective view from below. It will be noted that the circular projections 7 on the brick base are provided with shoulders 14 which have a horizontally aligned surface capable of being supported on the height control plinth 13 of a corresponding lower brick. Figure 13 is a perspective view of the third embodiment of the building brick in a form which is intended to be used as a 'base' brick 16 for a wall construction. Further views of the base brick are seen in the wall construction of Figure 14 and the wall cross- sectional view of Figure 15. The base brick 16 is formed without either of the projections 7 on the brick lower face so that it may be laid with its base in direct contact with a suitable foundation surface. The flower pattern 12 of depressions passes right through the brick body so that the bricklayer will be able to create in this hollow space a pillar of mortar. The pillar will thus extend from the foundation surface up to the projection 7 of a brick in the next layer of bricks in the assembly. The base brick 16 additionally carries a spacer 17 which extends outwards from the brick end face at one end of the brick. This spacer 17 serves to control the fit of one brick to a neighbouring brick in the assembly. It is only necessary for the spacer 17 to be present at a single end of the brick. The base brick 16 has been designed so that it can be laid either way up and, if necessary, the brick spacers 17 could be positioned one above the other at a joint so that at the end of a wall construction the presence of the spacers 17 would be concealed. The brick spacers 17 are provided with curved contact surfaces so tliat they will still be effective if required for the building of a curved wall. The base brick 16 also is provided with the height control plinths 13 which serve to support the projections 7 of a brick in an upper layer of the assembly. Figure 15 is the cross-sectional view showing the base bricks and the upper layers in an assembly. The base bricks 16 are seen to be spaced from one another on the foundation surface by the brick spacers 17. The next layer of bricks which is laid on top of the base bricks 16 will be supported on the height control plinths 13 of the base bricks. The height of the projections 7 on the next layer of bricks will ensure that there is a predetermined mortar height of some ten millimetres left between the upper edge of the lower brick and the lower edge of the upper brick. The provision of a suitable gap between the brick layers serves to enable an amateur bricklayer to construct a low wall which does not look out of place alongside an existing wall. The professional bricklayer would still be able to adjust the vertical spacing between the bricks in order to meet the standards of an expert worker. The building brick of the third embodiment has been found to be capable of metering the volume of mortar required to displace a suitable quantity from the cavity of the brick below. Thus, in constructing the wall of Figure 15, the builder will make up a mortar mix using, for preference, a mix where a plasticizer has been added. The plasticizer allows a reduced quantity of water to be used in the mix so that the brick faces will be kept cleaner. The builder will then make a first layer of base bricks on the foundation surface and fill the through hollows in the base bricks with the mortar mix. The bricks of the next layer are then tapped into place and as the projections 7 of these bricks enter the cavities of the base bricks the mortar mix will be squashed out and this material then fills the spacing between the bricks to form the mortar joint. The mortar joint can easily be finished by the builder by using a pointing tool since there is generally no need to add further mortar to complete the joint. The interlocking brick has been designed to enable a non-expert to construct structures that are compatible in appearance with standard brickwork. The design has means for providing a mortar joint of controlled thickness between the bricks. The provision of the depressions 3 which act to measure the volume of mortar required to joint two bricks gives a convenient test for the correct amount of mortar. If there should be small stones or grit in the mortar mix, these solid bodies will be able to fit in the loosely aligned spaces provided around the projections so that they will not interfere with the required alignment of the bricks. After the bricks have been laid, the bricklayer will as •usual tidy up the mortar line at the brick edges by pointing the joint with a trowel. This action serves to enhance the appearance of the joint and improve its weather resistance. The volume of mortar measured by the depressions may be arranged to provide a slight excess of the mortar volume to allow for a small amount of the joint mortar to be removed at the pointing stage. A professional bricklayer would still be able to benefit from use of the brick of the invention since this can give an improved speed of working, an improved strength of the resulting brick construction and facilitate the maintenance of clean faces on the laid bricks. Since the laid bricks have their own means for supporting one brick on top of another, it is no longer necessary for the number of laid brick courses to be limited whilst waiting for previously laid courses of mortar to harden. The interlocking bricks of the invention are also capable of being used with large brick and breeze block constructions and for tying in to old brickwork structures of traditional bricks. The foregoing description of some embodiments of the invention has been given by way of example only and a number of modifications may be made without departing from the scope of the invention as defined in the appended claims. For instance, the brick has been described as being of the standard housebrick size in order to be able to be compatible with conventional brickwork constructions. In an alternative embodiment, the brick could have different dimensions but the construction then may not blend in well with standard brickwork. Furthermore, although in the example given the polygonal cross-section of the projection is said to be a square, the use of this shape is not essential and a suitable alternative shape might be a circle, a triangle or a hexagon. The brick depressions may also if desired be of a polygonal shape, such as a square. In the embodiment of Figure 13, the height control plinth 13 provides sloping surfaces against which the circular projections 7 are seated to keep the brick of the upper layer spaced from the base brick. In a modification of this design, the height control plinth 13 could be provided with horizontal seating faces to provide support for the upper brick.