Gentlemen, thank you as always. You're right there are four pinholes in the bellows, or they're the ones I've been able to find! That's the trouble with having a fixed idea, I'd convinced myself it was the lens, and so wasn't thinking laterally.
So of course the next question, is it worth trying to repair the bellows? Or should I look for another one on the 'bay, with the risk that'll be leaky as well?