After careful consideration of all your urging me to "buy it", I've reached the conclusion that EASA with its preposterous permit has destroyed my dream. In the UK a 600 kg MTOW is only possible by agreeing to be subject to a regime which is much the same as for a certified aircraft. That puts aircraft owners squarely in the grasp of licenced engineers; no disrespect to such experts but it isn't in the spirit of affordable permit flying.
That leaves my only option as 472.5 kg MTOW flying on a BMAA permit and with full fuel, I could only carry half a passenger. So thanks for the advice anyway.