Feck! It never ends.
Managed to find 2 proper FV headlights to replace the generic Lucas headlamps. Points to note, the FV headlights are much better made and fit the fabulously complicated military headlight mounting much better. They are still available from Land Rover at an insane price (ÂŁ200 each)
But, fitted them and, of course, now one side doesn't work. Ah must be my rough handling of the bulb, whip it out, test it and it works perfectly. Obviously I have dislodged a wire somewhere. No, continuity and voltage tests and all good there, put the bulb back in and it doesn't work. Grab the wires and give them a good shake, an a flicker followed by a faint glow. The earth, must be the earth. Bridge it out, nothing. Test bulb again, works perfectly, swap headlamp units over and the problem swaps sides with it, swap the bulbs in the headlamps and the fault goes to wherever the bulb is fitted. Test the bulb again, works perfectly.
The only obvious difference is that the spring loaded plungers don't seem to be as compressed when the "faulty" bulb is fitted.