Sorry for the lateness of this response Wallo, I haven't got a clue what's in the gearbox yet.
By way of background , the guy I bought the bike from had owned it for 3 years (with the intention of restoring it), but hadn't done anything to it due to other projects. He bought it from the estate of the guy who'd taken it off the road, done some stuff to it, then popped his clogs taking it's secrets with him. Although I've ridden bikes for 40 years, this is my first Brit bike and first trials as well (it's very different to my old ZZR 1400 lol).
Currently I'm trying to determine what I've got (see next post) and where to go with it.
So now I've got a bantam rear hub, what sort of fettling does it require to fit to the C15, and what is the best size rear wheel to fit. Currently the back wheel is 4 00 17, but I've picked up that running a 4in rear can cause problems with the chain line .