If your fuses and bulbs are ok, its almost certainly the light control stalk by the steering wheel that's causing the problem.
The good news is that they are very easy to replace - you just need a phillips screwdriver and 10 minutes.
Unscrew the cover around the base of the steering wheel and take it off. The stalk then unclips and pulls out. Try wobbling it around a bit with the lights on and the main beam on - if its only slightly worn you might get the lights flicking on and off.
Go to a scrap yard and get a replacement if they haven't already all gone - its a common problem. Otherwise pay about £40 from a Vauxhall dealer.
If getting one from a scrap yard, note that there are two types: One has a single row of contacts, whilst the second has two rows. If you have the single row type, you can still use the two row type, but not vice-versa.
mike c, March 2010