Thanks OddJob - my pleasure in being able to contribute something to this excellent forum. And as you say I have indeed got new cush drive rubbers - their cost is minor compared to the primary chain!
In final stages of cleaning up crank cases, barrels, head etc. and will start putting all back together soon(ish). I have taken loads of pics as I pulled it apart (thanks to digital photography - would have been hopeless with film etc.!), so armed with those and a workshop manual and parts list (and this forum!) should help me get it right! Actually - I have used some Plastigauge to check the main bearings and they seemed well within tolerances - but as the plastigauge slipped a little in places - and I was rushing - I think I'll just do it again - and check the crank bearings too. I had no feelings that the bearings were worn when the bike was running - but I think I just want to re-assure myself!