Good to see you are going to take up one of the finest sports in the world! I bought myself a Marin Quake 7 a few years back for around £700 and it has lasted well - obviously with the abuse it take from me and my inept balance a few of the parts have been replaced. For 700 quid you can get yourself a damn good hardtail (just suspension up front) which will last for ages.
For good advice check out the
MBUK website and click on the bike test link at the top - there is also a valuable how-to buy a bike guide there. Got to agree with Tricky on the bike builds - Marin, Trek and Speciallized are all good quality.
Only thing left to get is a good crash hat. Used mine on several occasions and it has saved my bonce from some serious hospital action
