UK Net Guide (which the Answerbank is part of) has a useful guide to consumer rights. It correctly states the following:
"You do not need your receipt to claim your statutory rights, but it is extremely useful as proof of purchase. If you do not have your receipt, you may be required to produce alternative proof, such as a credit card bill or bank statement."
http://www.uknetguide.co.uk/Shopping/Article/Y our_Consumer_Rights.html
Some firms are very good about exchanging defective goods without a receipt. (e.g. Sainsbury's replaced a defective laminator for me, without a receipt, with no problems at all). Others can prove to be more difficult. However, it's quite likely that many Zavvi staff will currently think along the lines of "Stuff the company rules. I'm going to lose my job anyway, so here's your replacement":
http://news.bbc.co.uk/1/hi/business/7798973.st m
Chris