Tell the buyer to send it back post haste, that you'll refund the purchase price and initial shipping costs (which cost you nothing). It's easier, I found, to just be rid of the problem. Additionally, if they try to leave a negative feedback you'll then have grounds for answering it in that you immediately corrected their displeasure.
Often, they're just fishing for a discount thinking you'll avoid the hassle of refunding for a few dollars (or in your case, pounds) discount. I had this happen recently... problem solved.