Under the distance selling Regulations; you have a unconditional right to return these particular goods and expect a refund in full, (what you paid even if the delivery was free) providing you have correctly notified them of your desire to invoke the seven day cancellation right.
You can additionally expect a refund to cover your expense of returning these goods, unless you had prior notification when you entered into the contract... that the return of the goods would only be accepted at your cost.
Scroll down to the relevant rules in the link below subtitled Returns & Return of goods by consumer after cancellation.
Simply inform them that unless they honour the DSR's above and make a refund in full as per their legal obligation, that you intend to initiate legal proceedings through the money claim online service. Remind them that to lose this claim would also cost them the full legal costs.