The surcharge refund it done at our head office, so you can either post the old unit back, or drop it off at your local branch - who will return it to Wembley free of charge!
My experience with a reconditioned starter was you needed to get a return authorisation number from ECP [ Same mechanism as if you were just returning something unsuitable-- with you bearing the postage/carriage costs] before sending anything back. Otherwise was painless process with surcharge refunded to my credit card after about a week or so. No doubt Patrick will keep us right on correct procedure soon.