So I got back home and decided to find out how many valves I needed. They're about £15 each, so the bill could be ugly - I've heard of V6s bending 22 out of 24 valves before. There is a simple "redneck" test for this, that can be done in about 15 minutes. You need:
1) A head, with all pressure on the cams released (remove the followers)
2) A litre of (preferably red) diesel
3) A work mate or similar.
Place the head on the work mate, with one set of ports facing up, and have your diesel in a suitable pouring container:
Fill the ports up with diesel, close to the brim, but at least make sure you fill both valve headers:
Wait......and hope the other side stays dry. Inevitably some don't:
I'm working on the basis that if one valve in a cylinder is leaking, then its partner must have had a thump as well. So what is the score?
Cylinder 3 - both exhaust valves
Cylinder 5 - both exhaust valves
Cylinder 6 - one inlet valve, replace both.
None of the leaks are severe, so the hits were not that great.
It isn't a definitive test, but on the basis, it is worth attempting to do manual lapping, rather than sending them off to the machine shop. I'm in two minds whether I need to skim them anyway, I will consult the local specialist....