I have also experienced this approach from Comet when my iPod was faulty. I too was effectively told it was nothing to do with them and that I had to take the matter up directly with Apple. It is disgusting for these companies to even think they can try this on. As has already been stated, your best bet is probably to kick up a fuss with the store manager (perhaps escalating the matter via the complaints procedure, to head office if necessary) about the Sale of Goods Act making the retailer responsible, and not the manufacturer. I second what dx100uk has said about card provider being equally liable (assuming the item cost over £100) under s.75 of the Consumer Credit Act 1974.

Having recently achieved victory on my own underpayment, may I suggest you look at using the Extra Statutory Concession (HMRC's ESC A19 - http://www.hmrc.gov.uk/esc/esc.htm) as another line of defence against these numpties. ESC A19 basically means that if HMRC have been provided with the correct information, but have failed to use it for more that 12 months (which for the tax year 2010/11 means doing nothing until AFTER 6 April 2012), then the underpaid tax should be written off. In order to make a claim for this, it must have been reasonable for you to believe your tax affairs were in order (in your case the fact that your PAYE was being handled by your previous employer, should work). http://www.hmrc.gov.uk/esc/esc.htm