I've got that. I don't know if the wheels are the Vauzhall optional extras or what? If so, this website, http://www.tyre-shopper.co.uk/ts/search/VRMSearch.aspx, says they should be 255/35R18 Y.
The ones that are on now are 225 35 R18.
I'm not so convinced that the ones that are on now are the correct size. Quikfit had a look but seemed a bit unhelpful. Anyone any idea what size should be on?