Check the bulbs. When one headlight blows more current is channelled to the other headlight which can blow it if it's an old lamp. Best get them both checked to see if they have blown and replace asap as it's not legal to run a vehicle without working headlights.

Checking them tomorrow. But as they are seperately fused they must be on seperate circuits so I don't see how extra current could flow to the other bulb although there is unity through the switch. I wonder if the relay has burned out under the auxillary fusebox?

Mike, are you suggesting when one bulb blows this increases the voltage of the supply to the other lamp? The current through a bulb is subject to Ohm's Law, isn't it?