sales of goods act 1978 I think it is. you bought the goods from them, they MUST either
1) replace the item
2) send the item for repair (considering its a hard drive, won't happen)
3) offer an alternative item of equivalent value
4) refund you
5) offer you vouchers to the value of
you gave them an opportunity to respond, they refused. hard drive has been overworked is not a valid excuse. the product is covered under the sales of goods act.
if they refuse, contact trading standards, they will give them short shrift over this. in addition, if you bought it by credit card, contact your credit card company and get them involved. they will come down on pc world like a ton of bricks.
BTW. where in UK are you? I'm in leeds. can talk to you about this on phone for more detail if you are interested.