This morning I went to see Hughie and mentioned the problem to him. I suggested I should think again about heavier gauge wiring to the headlight as it sounded like a short circuit to him. (He didn't actually say that, but I knew what he was thinking!)
When I got home I decided to start with the obvious (well, it should have been!) and took the cover off the horn/dipswitch. Therein lay the problem! Every time the horn was depressed (I was getting a bit like that too!) the inner working came into contact with a couple of strands of wire leading to the dipswitch selector - resulting in a short circuit!
After trimming the wires, filing the terminal end down a few thou, and putting it all back together again - hoot, hoot... and the headlights stays lit! What am I like? All's well the ends well, I suppose. Now to book Little Willie in for