company not made refund-what to do?

bought goods form duvet and pillow warehouse in january. Some of the order was incorredt so returned the 3 wrong items. they have accepted that items have been received and claim refund made at the start of March. Still no money back on credit card. Have contacted the company - with only an email back saying 'thanks for your e-mail - your refund was made on the 11th March'

Have spoken to the credit card company who said to post a copy of the emails and they would look into it. I faxed them the infor they wanted at the end of march and have posted off copies a wwek ago. still no refund.

What are my rights and what should i do next?