If an item is defective when sold, you have the right to a full refund. You're not obliged to produce a receipt or to retain the tags.
However you might need to fight your case with the manager of the store. (Muttered threats about 'trading standards' and 'my mate who works for the local newspaper' might well be called for). Shoplifters know that if they nick a £30 garment they'll only get a fiver for it in their local pub. But if they put a hole in it and manage to convince the store that they bought it from there in that condition, they'll get £30 in cash. So store managers are naturally wary of giving refunds to people who can't produce a receipt.
Chris