A lot of choice at that price James....have you decided what sort of bike you need ? Full suspension or front shocks and hard tail ? Do you want a lightweight cross-country or a heavier free-style / downhillish one ? Theres a lot of top end brands that make each type, so its a bit of a minefield....Specialized / Kona / Scott / Orange I like myself, but theres loads more.
The Bike Lab in Poole (near the Dolphin Centre, opposite Barclays House) is a good place to spend an hour, and the guys there are very knowledgable.
My biggest tip is to put as much money as possible into the frame choice, next the drive train, next the brakes. That is, when deciding what offers the best value for money spec wise, remember that you can up-grade the ancilliaries later as and when, but the frame is the most expensive bit.
http://www.evanscycles.com/ good place to start
You can have mine for £3 k, Ive a bad knee.