Couple of rides you might want to consider
Excellent all round bike, quite easy to pick up cheaply 2nd hand. Here's one in Caerphilly BUT it's got a reserve and might go a little bit too high. Plenty more listed though
http://cgi.ebay.co.uk/SPECIALIZED-STUMP ... 4aa651f112
http://cgi.ebay.co.uk/Specialized-XC-FS ... 255ea81f24
This ones on Pinkbike with a knackered fork but seems a good spec and with £200 left over you could source some tidy forks for it
Try a Trek, few on ebay and Pinkbike
Gary Fisher Supercaliber, Hifi or similar
Supercalibers in my gallery, really light xc orientated FS, prob a little over budget
but you might be able to pick one up. Here's a Fat Possum listed on ebay at the moment
http://cgi.ebay.co.uk/GARY-FISHER-FAT-P ... 255f0af663
Sugar 3 on Pinkbike
Hows about Giant, Could try a Trance or this VT2
Marin float your boat?
I personally love Scott modern full sussers, but they can be expensive. For your budget you might even be able to track down a Spark or Genius with a little perseverance and luck.
With your budget it all depends if you have any brand loyalty. Most of us do. Sorry if I sound a bit ruthless here but with the current economic climate you may be able to benefit from someone's financial woes as they offload their pride and joy to pay for Xmas