When you purchase anything from anywhere you are setting up a contract of sale with the retailer. Under the sale of goods act you have 7 days I think (or 14) to change your mind. The retailer must take the goods back regardless of if they are faulty, not as described or whatever.
I am not too sure if they have to give you a refund or just a credit note.
But you definitely have this 'changing your mind' number of days to return an item under the law for a replacement/refund.
Some shops are better than others in their time limit and whether they refund or replace.
AFAIK
Maybe someone with a bit more knowledge can clarify this.
Greg