Yeah, stay away from Halfords, they do sell some good stuff (Kona) but they know nothing about bikes. Always go somewhere were they know about fitting and sizing a bike.
Keep an eye of these few sites, you may find a bargin and give you an idea what you can get:
www.wiggle.co.uk
www.wadescycles.co.uk
www.allterraincycles.co.uk
www.chainreactioncycles.com/