Well, that didn't go very well.
Drained oil, and to make sure I had proper access, drained the coolant as well so I could remove the casing entirely (water pump stuff is attached to the clutch casing).
Pulled the clutch assembly, can't see anything obviously amiss, the friction plates all still have plenty of meat on them, everything is straight and there were no obvious scrapes or marks to indicate anything amiss.
So, reassembled, new oil and coolant . . . clutch now slipping like a b@5$£!. Drained oil again, pulled it apart, checked I hadn't left a hammer or something in there. All looks fine. Reassemble, oil back in . . . still slipping. Slightest throttle and it just gives up.
I think I've set the lever that the cable attaches to at the wrong angle, as the clutch feels tight as well. It must be just starting to disengage even when the lever is fully extended.
It's a pain in the butt, as I have to drain it again (just the oil, the case comes away without draining the coolant). I ran out of time so I'm on the poxy scooter this week.