Plugs, Coil Pack, Lambda sensor or even as another chap MikeS recently found, coolant temp sensor. These are relatively cheap at around £10ish from Vauxhall. Another thing to look at would be the condition of the throttle body. Whip off the induction pipe and have a look. These can become gunked up. If so, remove the 4 bolts holding on the throttle body and clean out with carb cleaner or brake cleaner. Very easy to do!