Don't have a clue about the Xbox in particular, but I do know that under the Sales of Goods Act 1979 the goods must be 'fit for purpose' - ie, should not be breaking down within 11 months, and you have the right to expect the shop you bought it from to deal with it for you. Some shops can be pretty arsey about this, but you are within your rights to get them to deal with it and liaise with Microsoft if necessary on your behalf. You shouldn't have to pay postage, or incur any other liabilities yourself, etc.