If this is an international transaction then yes, you will be expected to pay for the return shipping. Otherwise, if the seller is in the UK and you have opened the correct type of case (item not as described) then eBay will give you a prepaid returns label to print and use at the seller's expense.
If you have opened the wrong type of case (a simple returns case is a 'change of mind') then you must leave it open. Then phone customer support tomorrow morning when the lines reopen and ask them to change the case over for you - that will be able to do that provided that you hadn't closed the case.
