Your rights still apply even though you do not have your receipt as other forms of proof of purchase are acceptable (in fact traders do not have to give you a receipt).
Go back to the store and tell them that you have proof of purchase and would like them to deal with the issue. If they won't ask to speak to someone more senior and if that doesn't work ask for the details of their had office/complaints procedure and use it.
Try these for some advice...
http://www.consumerdirect.gov.uk/