[RoryRover]

Thanks Simon

It looks like Westfield fasteners can help - they do an SS low head cap screw that looks ideal, I just need to check with them they have a 1.5 pitch version.

I have had a quick chat with a local MOT tester, and he tells me that provided the seat height is approximately the same as the drivers (in the raised position) and the seat was securely fixed, he would not have any issue with what I propose at MOT time.

I have just ordered 4 90mm lengths of 30x30mm mild steel square section that I will use as spacers to give a 30 mm total lift. Hopefully they will give an even more secure base than just using washers.

I will let you all know how I get on.