I bought a GT Aggressor XC1 from Halfords on the offer for £440 plus a £20 voucher to either sped on lights or lock, happy with the overhall bike apart from the forks not having a Lock Out.

For a budget look at these sites....

http://www.chainreactioncycles.com/

http://www.jejamescycles.co.uk/

Had a Gary Fisher Mountain bike in the past and for road/track riding it was great.