In reply to Green Porridge:
Thanks all! I traced it to a dodgy contact in the plug that connects to the rear light cluster. A bit of corrosion removal and voila! It works again. I suspect the plug itself might be a bit dodgy, so if it happens again, I'll give it a more serious going over.
Thanks again - it's good to know where to look first, before having to take the steering wheel off to get at the switches, with all the hassle that brings!
For the record, it's a 15 year old corsa