This is what you should do in order to get your charges back.
Write them a letter very similar to the one you send for the bank charges and change as apropriate. Ask them basically to prove that it cost £15 to send you a letter, bla-bla. Download the form N1 and fill it in and attach it to your letter. Refer to it as "Please find a sample of the County Court claim I intend to submit in the event you won't refund the charges". They will pass it to their legal department who are a bunch of morons and realising that they have no grounds for establishing a defence I believe they will