Many thanks for that comprehensive reply, Jaguar. I'm happy to go the same route as I've painted lots of stuff in cellulose, including Land Rovers and aluminium cars, using an isolator. If I remember rightly it's ok to use 2 pack primer and cellulose topcoat, but not the other way round.
Assuming you can still get cellulose, of course !
No idea why ours blistered first time round, I didn't actually paint it myself, but I'm sure the paints were supposedly compatible and the entire hull had just been refurbed and sandblasted by Ray Wright so there shouldn't have been any dodgy substances. Actually, I don't suppose it was sandblasted, but maybe something milder like walnut shell granules.
Still, summat was wrong - just hope we can avoid it next time ! In my experience if cellulose is going to play up, it does so straight way or not at all