So, Plan B - ramp extension time. I reckon I can add 2ft6 to the flat bottom and get rid of the metal plates. Should by a lot easier to learn on and a bit slower with the extra flat bottom.
Frame for the flat bottom extension is made:-
Its 2ft6 by 6ft. I used 25x38mm battens. The ramp flat bottom metal frame is 25mm box section steel and I didn't really want to add anymore height.
I also wombled two sheets of 8ft by 4ft 12mm ply from a skip in Swindon. One sheet is a bit gash but the other is OK. Hopefully I should have enough to fill the gaps where the metal plates would go (flat to the bottom of the transition). And then I think I'll skin it all with 3mm hardboard as the old ply is a bit rough.
Thats the plan anyway - we'll see how it pans out.