Well unless retail law has changed drastically over the past few years, under UK law you have a "reasonable" period of time to accept or reject the goods. If you bought it under 7 days ago, you are legally entitled to a refund on the item if you don't want it anymore, despite it being opened. You'll just have to put your foot down and they'll reluctantly end up giving in. For £50 it's something I'd be more than willing to do.
The store, however, does reserve the right to issue you with a credit note instead of refunding by your original purchase method.
