Finished the Transom Rebuild!!!
Wet the ply with epoxy, then with thickened epoxy spread with a grout spreader all over. used slow hardener with the epoxy.
Clamped and left for 48 hours. with 2 m12 bolts through where the engine will bolt on.
then beat off the 4b2 with a lump hammer (mental note use wax paper between if you ever do this again...)
8 layers of 300g woven roving, 1 2 then 3 inches into the deck over the top and all the way down to the keel on the back, fast hardening epoxy (20 minutes laying up time) left for 4 days to go off.
2mm thick 90cm wide 3" deep with internal gap of 1 1/4 inch stainless cap, engine top mount and struts bolted through with m12 bolts
mounted the engine
have now had 2 hours bumping around in chop to no ill effect,
thanks to all those who offered advice, cost me £150 saved me £600.