In reply to JamButty: The forks are often a limiting factor on budget bikes as decent forks alone cost upwards of £200. I wouldn't spend money on anything with Suntour XTC, RST or Rockshox Dart forks. Decent ones to look for are the a Suntour Epicon, Suntour Raidon and Rockshox Tora or Recon. Rockshox XC30 are OK.
The cheapest decent bikes from new are Vodoo Hoodoo, Boardman and some of Halfords Carerra range, so chances are one of these would be good value secondhand. Try doing an ebay search and see if you can find one locally so you can have a look at it and try it for size. (As all are sold by Halfords you could nip in there and try out for size).