This is what "Which?" has to say:
"If you buy a product that turns out to be faulty, you can choose to 'reject' it: give it back and get your money back. However, the law gives you only a 'reasonable' time to do this – what is reasonable depends on the product and how obvious the fault is. However, even with something like a car, you usually have no more than three to four weeks from when you receive it to reject it."
So it would seem that Carphone Warehouse have the right to choose to get it repaired or replaced at their discretion.
It goes on to say:
"Under the Sale of Goods Act, the retailer must either repair or replace the goods 'within a reasonable time but without causing significant inconvenience'. If the seller doesn't do this, you are entitled to claim either:
reduction on the purchase price, or
your money back, minus an amount for the usage you've had of the goods (called 'recision')."
http://www.which.co.u...oods-act/your-rights/