To complete the story, last weekend I extended the opening at hte back of the headlamp shell to get the routing of the speedo cable nice an smooth, without any sharp bends or kinks, photos attached.
I think this modification is better than the standard cowl headlamp shell, in that I can remove the headlamp complete without disturbing the speedo cable. The speedo cable appears to pass throuth the standard shell, which would mean disconnnecting the caple if the shell needed removal.
During my excavations inot the back of the shell, I came across a previous alteration (a cutout of the tinware) which was thouroughly repaired by the firm I used to paint all the parts. I suppose he thought he was doing a good job restoring and repairing the shell to what he though was original. I never cottened on that the cut out was so important, and my restoration of the bike took so long that I forgot how everything fitted together under the cowl. The bike was dismantled around 1982 and sat in boxes for a long, long time.
The speedo seems to be working properly, having clocked up about 85 miles so far, and the machine has passed its MOT with flying colours. He never looked at the action of the clock, but did take the mileage reading.
I can now build up some miles on the road and sort out some other minor niggles, eg a bit of clutch drag making finding neutral a bit of a chore, a stiff clutch action and an occasional misfire at low revs (probably points setting imbalance between cylinders).
I'll look at the points tomorrow, and change the oil before our bike club's (the SCMC) annual summer run on sunday. We start at the Baiglie Inn between Glenfarg and Bridge of Earn (10 for an 11 start) south of Perth, Scotland, for anyone interested in a nice run through the Ochils and returing for something to eat at the pub.
Thanks again for all your advice, looking forward to further helpful positings. If only I found this site before!
John