Paul M

Cheers mate. The fact that there was no swarf or anything else that shouldn't be there in the oil seems to imply that the bottom end is OK. And the noise was definitely from the top end on the front cylinder. So I expect that replacing the front cylinder head (or maybe both heads just to be safe) would fix it... no guarantees of course until the head is removed and the cause of the problem discovered.

I'm getting tempted to try the fix myself if the bike doesn't go soon, as I expect I'd make a lot more money that way. But again time is going to be an issue and I'm horrendously bad at estimating how long a job is likely to take.