Thanks, Chris
After more fiddling about with the car, I have have taken my generator to Classic Dynamo and Regulator Conversions in Lincolnshire. The dynamo will be rebuilt “better than the original” and the voltage regulator will be converted to electronic. I just wanted to be absolutely certain that the fault is in the wiring before I undertake the daunting task of removing the speedo.
Thanks again for your help. I’ll update my post when I get the parts back.
Ian Stroud