Okay, slightly better today, so here goes.. slowly.
Thanks for the input everyone.
I pulled the plug and tried to roll the bike. No good, the back wheel just drags. Plug looks good though. Got the cover off the gearbox. Apart from some crusty crud in the bottom, doesn't look too bad. The flower nut seems loose. When I swing the clutch arm in, I can push the underlying plate down enough with my thumbs to almost be able to turn the flower nut. Pretty sure that's not right. With the beast on the stand, if I pull firmly on the wheel (plug is still out) I can make the engine turn once or twice, but it's very hard. The teeth on both gears look alright to me.
I'm thinking to take the flower nut off next, and see what lies behind it. I'll add another pic when I get that far. I have to keep coming in for a few minutes now and then to warm my hands up, it's a nice but cool one today.