Right, first of all thanks for those who helped me with the Wii question. I had some online leads but went to the massive Tesco Extra store down the road on the off chance and they had one in stock, so bought it there and then.
The tricky bit of this is that my mum has already bought MrSC a Mario Kart game for DS. She bought it online so can't get it changed in time for Christmas. I've said I will go out and try to exchange it - just saying that my mum bought my husband a present but got it for the wrong console, I don't have a receipt, will they exchange it and I'll pay the difference which isn't far off the truth.
I've done similar (where the story is slightly more true!) before but can't remember where - I think maybe HMV. Has anyone done this recently and can advise? The game will be sealed in it's original wrapping etc. Can't ask my mum to send it back as a) it probably won't come back in time and b) I think she's already pretty annoyed with me for being a pain as it is!