I had been with Freeserve for nearly 8 years, and I suffered this same problem in July "The time of the change over - Freeserve to Wanadoo". The old payment method was via a company called Worldpay, but Wanadoo now manage their own payments but it seems they have lost some customer payment details "Mine Included".
I have incurred charges of £27.00 trying to resolve this issue, and am currently seeking compensation from Wanadoo for my loses.
I am know longer subscribed to them as an ISP due to the way they have dealt this occurance. As a customer the service I received from the Customer Service Advisors and Mangagers alike was shocking at at all times they have failed to acknowledge any responsibilty even when I was told that "I had changed my credit card details".