top edges of a modified carrier beam 212 to engage within lateral groove formations 212 6 provided in the side walls 212 1 212 2 of the carrier beam 212 it will be observed that the modified carrier beam 212 is otherwise similar to the carrier beam 12 depicted in fig1 3 the modified fixed lever 316 depicted in fig9 again has an upright arm 316 3 with an off set upper port 316 5 and an aperture 316 7 instead of a transverse limb the lever 316 has a pair of perpendicular attachment flanges 316 4 and 316 5 to be arranged to straddle a modified carrier beam 312 the flange 316 4 is attached to the beam 312 by means of a screw 312 6 it will be observed that the carrier beam 312 is again very similar in shape and configuration to the beam 12 depicted in fig1 3 except that it does not have the groove formations the modified fixed lever 416 depicted in fig1 13 and 15 comprises an upright arm 416 3 and a transverse limb 416 4 the upper part 416 5 of the upright arm 416 3 is again slightly off set as explained above and an aperture 416 7 is provided for connection of the fixed lever 416 to a free lever 418 and to a carrier element 414 the transverse limb 416 4 is formed at its free end to present a generally inverted v formation 416 2 having an inwardly directed lip formation 416 1 at its extremity it will be appreciated that the v.