Thanks Swarfy. I must admit the earth from rear light to the battery + earth is a very recent addition (actually only 2 days ago). Over the past few weeks my bike has been up on it's work platform for a fork overhaul, a new rear sprocket and while waiting for replacement parts I took the outer and inner covers off the gearbox to check if anything was amiss after detecting a slight difference in sound in certain circumstances. Pleased to say (to me) all looked fine but while it was out I did renew the mainshaft bearing and bushes in the inner cover. During this time I also stripped and resprayed the fork legs, trousers, front mudguard & its fittings. Then the rear mudguard and number plate, finishing it all this weekend, only to find the rear light and the stop light wasn't working. Bearing in mind that, in addition to the rear light, all the wiring inside the nacelle had also been disconnected. I was sure I had connected it correctly but removed the headlight to check. All was ok and I was puzzled. I eventually traced it to the rear light not having an earth, so I added the new earth wire and all worked fine. Previously the earth must have relied on a bolt from the light fitting making earth through the mudguard and, of course, I would have painted over it. Checking an original wiring diagram for my bike's year there was no separate earth wire from the rear light and it must have relied, from day one, on making an earth through the tinware, frame and to battery.
Now waiting for my son to come and help me take it off the work platform.