Job complete. Took about 15 minutes. A bit fiddly, but you should be able to work it out. It is a sealed unit, so no repair is possible.
Part number 9181010LSC
Item cost �17.35 from my tame back street supplier. It will probably be a bit more from a franchised Vauxhall dealer, say �20. If you can get one from a breakers for �5 it might be worth the risk, but you may have to do it again soon after.
5 screws to remove. 3 are deeply recessed in the shroud beneath and behind the wheel. Cross head screwdriver. If it won't undo you are scraping plastic beside the screw even though it feels like it has engaged.
Once they are all out turn wheel 90 degrees to reveal a black plastic screw cover facing you. Prise off the cover and remove the screw. Turn wheel 180 degrees to reveal the other one and remove.
Pull off both top and bottom shroud, trying to remember which way you pulled them out. The fiddliest bit is putting these two back!
Slide out the switch assembly. I think there may have been a catch to release it or I may have prised it out with a screwdriver. Pull off the electrical connector. This time there was some sort of plastic catch to lift, holding it in position.
Fit new one and put it all together again.
I think I struggled to get the bottom shroud back in as I tried to put the furthest part into the hole in the dashboard first. It went in by offering the edge to the steering wheel first. Top bit went in dashboard side first.
You have to fiddle to get flexible trim around the igntion lock and both stalks in, but it will all go.
Good luck. You,ll be pleased with yourself afterwards. (Apologies to any qualified vehicle technicians, you are probably laughing your socks off at this).