Hi Flimbo,
I'd go for an earth fault, have had similar with left hand headlight in the past, worked dipped though dim, went out on full beam.
The dipped left headlight was basically earthing back through the indicator switch, or somehow through the full beam circuit, so when I touched the indicator stalk, the left hand went out, sometimes both lights went out, scary on unlit backroads :shock:
Get yourself a 12v electrical probe tester, it looks like a sharp screwdriver that has a 12v bulb in the handle, and has a crocodile clip on the end of the cable. Cost around £3 from a car place, or car boot sale tool stall.
Or you could run a separate wire from the earth on the indicator to the -ve on the battery and see if the circuit works correctly.
It could be a fault on the front, or rear, or possibly both (black cables), it may also be worth checking the joint earth on the indicator warning lamps (black&red), though I suspect this will have a good earth as it's in the dash.
If you're unable to locate it as an indicator fault, don't rule out a faulty earth on your headlights.
Regards,
Rob.