You are all on the right tracks :winky:
Firstly, has the vehicle been fitted with the breather modification? This will be a black collector box at the front of the engine (on crankcase breather pipe). Vauxhall were aware of this problem and introduced this modification. Its only £10 from Vauxhall, so if it is not fitted, buy one. This will not cure the problem but should help prevent it in future. Also, make sure the engine oil is regularly changed. Dirty oil will only promote this problem.
To solve this problem, firstly clean throttle body thoroughly with carb cleaner. This includes the butterfly (especially on edges). If this is sticking slightly it will have poor idle and will stall.
Next, check the the little hole at the back of the throttle body (above butterfly) is not blocked.
Next, as the guys have said here, remove idle valve. Split and clean thoroughly. Remember to get a new gasket from Vauxhall before re-fitting.
Finally, take off all breather pipes and clean out thoroughly. clogged up breathers tend to cause the erratic idling.
If you still get problems, PM me and I will let you know the other likely cause :winky:
9/10 if you do all of the above, it should be fine