Just having a tidy up and found this,i think it came with some track bodywork via flea bay,think its something off a seat unit but what is it exactly..?
It should sit underneath where the seat pad will be. There is actually a gap in between the frame and the seat unit and this will sit there with the two grooved parts at the top sitting on the frame. This will make the seating area more solid. At least that's what it looks like anyway