You can't charge a restocking fee, and if they've sent you something faulty or the wrong item, I believe they have to pay postage also.
I believe the retailer can make some sort of restocking-style deduction, depending on the whether the item has been opened/used. One of the OCUK staff made a post about it (I've not checked it's legally correct, but it seemed reasonable);
http://forums.overclockers.co.uk/showpost.php?p=26942736&postcount=22