If the goods are faulty I don't think you should have to pay
See:-
Sending the goods back
Unless you have bought the goods under a distance selling contract, you won?t have any legal right to send the goods back if you have simply changed your mind. You should refer to that retailer?s return policy and will probably have to pay postage. If the goods have arrived damaged, and you have been offered requested a repair or a replacement, you should not have to pay any further sums of money in this respect. This includes labour, materials or postage. The repair / replacement should take place in ?reasonable time? and you should not experience any ?significant inconvenience?, while you are without it/them. For more on this, see our section on Sale of Goods and Returning Damaged or Faulty Goods.
In all respects, you will have greater rights if you had bought them under a distance contract (online, mail order). This is because you didn?t have the opportunity to examine them before they were shipped ? to see if they are:
Of satisfactory quality
As described
Fit for intended purpose
Appropriate to your requirements
With this in mind you will have a 7-day cooling off period, during which time you can cancel the contract and get your money back in full (including delivery charges). Unless otherwise specified (for certain types of products), you should be able to send the items back for free, and not incur any additional charges such as a restocking or admin fee. Although many retailers will request products to be returned in a saleable condition in their original packaging, they cannot make it a condition of giving you a refund. For more on returns and refunds under a distance selling contract, see our sections on distance selling and buying online.
from here