I'm guessing that the most likely cause (before I go exploring valve stem oil seals, rings, etc) is the slipper clutch diaphragm.
I had a plan to get one "in stock" before investigating to save having to open it up twice (once to investigate and once to fix it).
Having looked it up, it seems to be about 5 times the cost I was thinking it might be .... so I guess the plan to just get one is not so sensible.
...... that is unless anyone has found a cheap source ......
I doubt it, but just in case .....
