Under the sale of goods act you are entitled to a full refund, inform the shop that. "under the sale of goods act I am entitled to a full refund so please arrange to have the goods collected within 14 days and refund the costs" also a registered letter to the shop or their head office on the same lines would help, if they try to deliver a replacement refuse it. Should they try to get out of this,inform them that you will pursue the claim through the small claims court.