Just followed this and it's easy enough.
I didn't fancy removing the bell from the shaft myself as I was worried it'd be too loose when replaced, so after cleaning with petrol I used cotton-wool buds and metal polish spinning the buds between the shaft and the bell. Then made sure there was no cotton fibre still in there and clean with petrol.
I tested it with a continuity tester and it's working fine.
Only thing I'm slightly concerned about is the mention of 'epoxy' as the seal on the surface.....I used Araldite, but I'm not sure the original is actually epoxy glue, as it seems to come off surprisingly easily with a fine screwdriver....in fact some of mine was already missing.
I'll do the one on the spares bike next.
I also took the advice offered here and changed the Allen head bolts for hex....so the oil screen can be removed with the engine in the frame!
I had installed the neutral switch wrong in the photo, so I re-fitted it correctly....I put a smear of Vaseline on the body so it doesn't stick in the oil seal like before, and copper grease on the threads of all the bolts and the cable screw.

"
The Moving Finger writes, and having writ, moves on,
nor all your Piety nor Wit
can lure it back to cancel half a line...
so....probably best use Tipp-Ex in future?"
1977 GL1000 in bits
1978 SR 500 Yam
1995 Harley Sportster/Buell street-tracker
1992 Grinnall Scorpion SC3 prototype
2002 Caterham SV 220 Evo (4 wheeled motorcycle)
Hal's Nalbum