Virgin Media have overcharged me by about �200 but they refuse to refund me the money they owe what action can

Virgin Media mistakenly charged me a twice for about 10 months for an internet account. I didn't notice for a while becuase they charge one amount to my credit card and one amount to my bank account. They now keep not returning my calls or answering my letters when I try and get them to refund my money. What can I do?