Hi, thanks for the replies, boys. I phoned Dave Renham on Wednesday and had the required parts list in my possesion on Thursday. Great advice from Big John and superb service from Bultaco UK. Bob, you were absolutely right. The speedometer drive does indeed form part of the wheel spacing on the rear axle.
I have just bought a pair of snail cam chain adjusters off ebay and am now in search of the speedometer drive and distance bush. Any suggestions anyone?