After all that I think we had the same thing! I don't drive the missus car very often, I was sat in it last weekend with the fuse still out and decided to try putting the fuse back in to see if the problem had fixed itself. Surprisingly it had! After reading this I think I may have been playing with the headlight switch while waiting in a car park and might have pushed it back in sub consciously??!?

Yeah I feel stupid too!

No, it's less obvious that that, the head light knob which you rotate to Side lights on then main beam can also be pulled outwards. I didn't realise this and this is what makes the interior light stay on permanently.

Strange idea from Vauxhall!