The law requires that you must provide customers with details of their rights under the Distance Selling Regulations 2000, including their right to a full refund. As long as you specifically provide such information and do not attempt to contravene the Regulations in any other way you can then add in a contractual condition that the customer must pay the return carriage.
However restocking charges are ILLEGAL, so your notification (if any) of the customers rights under the Distance Selling Regulations is no longer valid. Where you have not provided correct information in relation to the customer's rights, you ARE responsible for paying the return carriage costs (as any contrary statement within your trading terms is automatically invalidated).
Nobody should be trading on the internet unless they're read and complied with this:
http://www.oft.gov.uk/shared_oft/business_leaflets/general/oft913.pdf
Chris