Well I finally got round to heaving the engine back into the TS250 yesterday & after cleaning the contacts in the red connector block under the seat and also the points it spluttered into life (after attempting to break my shin on numerous occasions), main bearings rumbling away nicely.... B*LL*CKS!
Oh well, at least the top end's nearly like new. Another bonus is that I've hopefully found a spare 3 speed lump which could be fitted whilst I do the bottom end of the original. Until I build the cash for that & a new set of mains etc. I'll just get it tidied up, stick a chain, sprocket & silencer on it and ride it round for a while to find any other points that need sorting.
I still think it was a good swap though & reckon it'll make a nice, tidy unrestored runabout for a couple of years before I decide/need to do a full on refurb to make it perfect.