Personnaly i would steer well clear of buying a referbished (or second hand) machine.
I bought one from pc world a couple of months back, and within 5 days it developed a fault, sent it back. they said they fixed it, but it was still broke, so in the end i gave up and bought one from BHS direct of all places. The only fault i've had so far was a dodgy hard disk, but there common faults, and BHS repaired it lighting fast, they even cleaned it!