Well, problem found I think! I last rode it a couple of weeks ago, dragging quite badly but the after a couple of miles it started slipping....so much so that when I tried to start it the next day you couldn't kick it over...Today, I tried it again and clutch wouldn't release at all, so although I haven't got any new parts yet I just had to look and see...
First took out the case level bolt and a good bit of oil came out (have checked a few times and doesn't seem to be wet sumping btw), then drained it....over a pint of thick oil....gear oil I think. Way too much I believe? (Still haven't got round to buying a manual!!)
The clutch seemed to be lifting evenly so I took the springs and pressure plate off, still solid though, you could turn the engine over using the clutch nut (9:1 comp and plugs in), began hooking the plates out and they all seemed almost bonded together, some I had to force apart with a knife blade... you could still turn the engine with one of each left! Also the insides of the basket coated in a red rusty coloured thick grease and lots of pitting on the plain plates....
Should mention here that I bought the bike recently from someone who had had it rebuilt and then not used at all for about 4 years, all of the clutch parts seem to have been replaced.
So, I'm thinking that the general lack of use was causing the dragging and then as it got soaked in oil as I rode it, it began to slip, leave it for a while...the plates stick themselves back together again and so on....?
I was fully expecting to see the clutch hanging off and maybe some damage to the mainshaft so fairly pleased really, my plan was to fit a Pearson clutch to this bike and use the existing one on a bike I'm building from parts....is it worth attempting to clean the friction plates in some kind of solvent? I've tried cleaning up the plain ones and they are going to be OK I think...