I would suggest a known brand like Seagate, Hitachi, Samsung, Western Digital. Have a look as some offer extended warranties on the drive. I havn't had much luck with Maxtor in the past either.
PC World are OK with boxed products - as they just resell (wouldn't use them for a service though) - beware the cheaper independent shops as some do rely on cheap imports (ok so they all are imported but you get my drift). I tend to buy my drives from a shop rather than web - as one drive with your local postie/courier could easily be dropped.
Maplins can have the odd bargain too.