So it looks like I'm taking the circa 1% chance of death or stroke to have a mitral valve repair, medication not being an option to deal with the size of the leak (as already intimated). Let's just say it's definitely not keyhole surgery.
Bits of not-such-bad news: the consultant has said it's fine to travel to France by plane/train in the meantime; that Autumn might well be possible for surgery (and no particularly elevated risks in the meantime if I'm sensible); that the consultant will be writing to the best mitral valve repairer he knows (who happens to be at the Royal Brompton, and who repaired the valve of a good friend of his younger than me, who's regained all lost athletic capacity), and though my CT scan isn't perfect (a bit of furring), there's nothing serious to warrant undue concern.
All the same, not quite what I thought I'd be doing in the first year of my "Who knows what's round the corner?" semi-retirement. Ho F hum.