Usual source of problem is bad switch (the rotating one). they take so much current that the contacts burn.

>>There were several ‘high power’ wiring looms offered over the years, which used the original circuitry to simply switch an additional relay to each headlight, and provide a different route for the headlamp supply.
>>I’ve seen a few, and they were a fire hazard due to really crappy quality. But the concept is sound.

Yup-I bought Phil Payne's old ur and surprisingly it had a shitty loom with no fuse and heavy duity wires from the alternator across the engine bay to up by the radiator.
binned that!

Somewhere over the years i picked up a much better one. Fused right next to the supply from alternator.
Haven't fitted it as my lights are fine. good earths, clean switches, clean bulb contacts.

Easiest thing to do is try a better earth jumpered in from the back of the headlight bulb. If that fixes it, you know you need to follow the earthing back and fix it.
If it doesn't, then its on the power side and a wiring diagram woudl REALLY help.

It only takes one rotten connection and the whole thing would be poor.

>>Each bulb is drawing several amps, which will also manifest as heat in other areas, like the headlight stalk, and the fuse/relay box.

Good call-For a similar issue in a house wiring, i used an IR camera to see what was producing heat, and tracked the issue that way.

Same could be done here, with JUST the headlights on use an IR camera to see what is getting hot!