The simple answer is to not take this sort of sh*t from Comet (as an ex-employee) i can confirm that not only do the management have no idea of consumer legislation but the company also directly refuses to acknowledge there comitment to the consumer as set out in the sale of goods act).
It is your legal right as a consumer to have this matter resolved under the terms of the contract of sale when you purchased the item. This means, very simply, that the retailer has a legal (and written - in the form of you receipt) contract to provide the stated warranty to you. If they are unable or willing to do so then they are in breach of both the "sale of goods and services act" and of Trading Standards Regulations - both the Trading Standards website and any site quoting sale of goods act advice will confirm.
The best trick is to stand your ground in the shop. Make the manage come out, make sure it's in the middle of the shop (in public) and clearly quote the law to him. Make sure you speak loudly and clearly enough for every customer to hear but don't shout or get aggressive. It's about the worst advert for his shop that he could imagine, he will know you are well within your rights and (in the long run) it will be much cheaper and easier for him to just give you your money back and usher you out of the shop then to continue to lose business as other customer leave.
From personal experience i would recommend that you then take your money to somewhere like John Lewis or Richer Sounds - either way you know the service will be much much better.
Don't be mistaken by thinking that this is an objectional manager at your local Comet - it is a standard business practice of a lot of the 'out of town' national retailers.
I had a very similar situation with a pda at Staples and eventually printed the Sale of Goods Act out and handed it to the manager (i highlighted the section that states the contract is between the retailer and the consumer - the only contract the manufacturer has is with their customer, ie. the retail company). Then i asked him for his written refusal to give a full refund so i may sent both to the small claims court and the local paper. You would be amazed at how fast the till opened and the cash was in my hand - he didn't even care that i had paid on visa initially! A little harsher than i would recommend but i was not in a good mood by this point.
If all else fail then take them to the small claims court via
www.moneyclaim.gov.uk and get all you money back.