comprises a respective horizontally extending frame part 20 21 each having an inner end portion integral with the upper end portion of a vertically extending frame part 22 23 respectively each vertical frame part 22 23 has a lower end portion integral with a respective lower connecting part 24 25 each having a respective terminal end 26 27 fig5 each horizontally extending frame part 20 21 also comprises an outer end portion integral with a respective outer connecting part 28 29 connected together at 30 by welding and reinforced by a plug 31 located within connecting parts 28 29 at their connection 30 each of the horizontally extending frame parts or members 20 21 are elongated spaced apart and lie in substantially the same horizontal plane each of the vertically extending frame parts or elements 22 23 are spaced apart and lie in substantially the same vertical plane the outer connecting parts 28 29 connect together the elongated horizontally extending frame member 20 21 at their outer ends and the lower connecting parts 24 25 together with other structure to be subsequently described connect together the vertically extending frame parts 22 23 at their lower ends trough shaped skin 16 is composed of two separate halves joined together each half comprises a respective side portion 90 91 integral with a respective bottom portion 32 33 integral with a respective upstanding lip portion 34 35 lip portions 34 35 are joined together along their length as by welding to form a central reinforcing.