I removed the ignition switch and opened it up.
On a previous check a while back, after opening it up and finding some wear on the saddle dimples, I decided to turn the saddles round, as one dimple on each saddle had more wear than the other.
This appears to have been a mistake which lead to the fault of loosing the headlight and battery charging.
When I opened up the switch I could see the witness marks in the dielectric grease on the contact base plate that had been left by the dimples, and this showed there was minimal contact with one dimple.
When I removed the saddles and looked carefully at them, they were not symmetrical, as they first appeared to be; one end was slightly longer and the dimple further out from centre. This was enough to cause the fault, although it took a while to materialize.
I refitted the saddles correctly, put the switch back together and refitted it.
A run of 90 miles later and no faults seems to have proved a success.