I'm sure there would be no probs with that. Maybe would take a bit longer to have a refund issued but I can't see that sending in the two would cause any hassle [although as ever I am open to correction!].
Stick a contact number on your letter, ask them to contact you with any queries that might arise/if they need further info. and if you haven't heard back within a reasonable period give them a buzz.
Ultimately, if you're due money back you'll get it back and sure the worst they can say is everything's in order & you're not due a refund!