Ok, stripped clutch down today and found the shaft nut was loose after all.
The thing that bothers me is that it seems to have loosened quite a bit against the locking washer. When I released the locking washer, the nut tightened up a lot (a few turns at least). Can't see how this could happen unless it's been forced round, but the washer doesn't look as if it's moved.
Think I'll reassemble with some Loctite on the thread and see what happens (it was belted up tight originally).
Couple of things though:
1. Is it worth replacing the roller bearings - they seem ok but no idea how old?
2. As in the picture of the clutch centre below, what are the internal threads for??
3. Because all the oil had wet sumped, I managed to drain most of the engine oil through the primary screw holes.
4. Primary chain has some tight links - time to replace I reckon (only done around 12,000 miles though).
5. Any tips/ideas on why the nut loosened - has happened before. Does clutch centre need to be tighter on the taper?
Checked crank for movement Bergs, tiniest amount of endfloat, but not visible and nothing like before (no up and down movement though). Gearbox mainshaft has a little bit of up and down - can't remember if this was a RM special or not.
Comments and advice always appreciated fellas.