I have now traced the fault (initially) to the small light in the headlight shell. I took the headlamp off and discovered that the reflector is live and the rim shorts against the nacelle. The bayonet fitting has metal to metal contact with the reflector, but I think that is standard. When I remove the small bulbholder from the reflector and refit the headlight, all the lights work ok. I tested the bulbholder off the bike, by hooking it up to a spare car battery and it lit up ok (as it does on the bike) and when I touched it to a steel cabinet it continued to light and without sparking - unlike when it is fitted to the bike (when it sparks the headlight against the nacelle and kills all the front lights, including the speedo/ammeter lights and the tail light). As I said, the indicators and brake light are unaffected. Clearly something else is not right and I am now at a loss as where to look next!
This is so weird!As I was still having no luck tracing the fault, I tried swapping the spade terminals around on the pilot light just to see what happenned - and BINGO - problem solved! Or is it??? Because during my checks I carefully marked the two spades to ensure they went back the same (same way that they had been for four trouble-free years in fact!). It's as if the polarity has somehow been reversed for some reason (and no, the battery has never been fitted the wrong way!).So it may (or may not!) have been fixed now. How could this possibly be?
Try looking at the wires where they go into the socket. It sound like the power wire is moving around and making occasional contact with the shell.Bare