I did enquire into upgrades, but there's a 2-3 month waiting list for the upgraded part - nVidia 8200GTX instead of 7900GTX, and it would cost £325, so I decided against that.
I've opted to have it picked up at no cost tomorrow for repair back at the factory.
So I'm slightly flabbergasted, in a good way. I think it's because it's an XPS model - a top of the range one that they made a lot of fuss about being reliable and having good support etc. There was even a premium support line number to call on Dell's site.
So now I feel a little stupid for my wasted trip to Oxford today, but happy that it's going to get fixed :)