Under DSR you have the right to return goods for ANY REASON within 7days, even if you have simply changed your mind, whilst this is a pain in the rear end for retailers there is nothing they can do about it.
Unless specifically stated in the retailers terms and conditions then the retailer pays for the return costs, however if they have it set in the Ts & Cs that the customer is liable for the return fees then they can charge the DIRECT costs of the return, this is just the cost of any courier or post and packing to get the item back to the retailer, they cant add any admin costs and they cant add any re-stocking fees.
They have to refund you your full cost price including your original P&P costs so the full original price you paid with the only deduction they can make being the cost to get the item back.
Its different if the item is faulty but if you're quoting DSR for your reason for return then the above is what applies.
I hope that helps!