Just to clarify a legal point, if the order was placed by 'distance means', e,g mail order, by phone, over internet etc, then they will have 30 days to supply the item and then have a further 30 days (so 60 in total from order) to refund your money before they are breaching the Distance Selling Regulations.
You can certainly put Ethel's suggestion in your letter but bear in mind no breach of legislation actually occurs until after the 60 days are up and you have had no item or refund.
Practically, a call to the credit card co might resolve this if your letter fails.