lower faces of adjacent modules the lower face 15 in use of the module includes secondary raised portions 56 a which interengage with secondary recessed portions 57 a on the upper face of adjacent modules when the modules are stacked this interengagement of the secondary raised portions 56 56 a and secondary recessed portions 57 57 a prevents lateral movement of the stacked modules along the length of the ridges referring to fig3 a it can be seen that the lower face 15 of the module is narrower than the top face 12 of the module the side walls 33 of the module are generally curved in cross section as can be seen more clearly on fig1 2 and 5 referring now to fig6 a there is shown a peg suitable for linking adjacent modules to each other the peg 61 is generally i shaped in cross section as can be seen by reference to fig6 c the peg 61 is tapered along its length as can be seen in fig6 a the peg 61 is sized such that it fits into the receiving portions 31 of adjacent modules the peg 61 further comprises a releasable locking mechanism in the form of two resiliently deformable pins 62 the pins may be integrally formed with the peg 61 which is produced from plastics material or alternatively produced from material which is more readily resiliently deformable such as sprung steel which is then attached to the peg 61 by known means the peg joins the modules together by interlocking with a receiving portion in the form of a recess 55 as shown in fig1 3 and 5 the recess narrows from the.