A constructional unit comprising: a support frame which is O-shaped in plan bounding an open central region, the support frame including first and second members which are each U-shaped in plan, each member being in the form of a base frame from which extend two side arms; the first and second members having their side arms telescopically engaged to define sides to the open central region of the O-shaped support frame; the telescopic engagement providing for- the spacing of the first base frame from the and second base frame to be adjustable over a range of distance; and the temporary securing of the first member to the second member at a predetermined spacing; the base frame of the first member being adapted for location at a first level at a first working location; the second member being adapted, at a position remote from the first member, for location at a second level at a second working location by means of at least one leg pivotably attached at or near one end of the, or each, leg to the base frame of the second member or to a side arm thereof; the opposite end to the one end of the, or each, leg being adapted for location on the second working location; the second level being offset from and below the first level.