Dear Sir/ Madam,
Reference: [account/ card number]
I write further to my telephone call on [date]
On [date] my [details of card] was [stolen/lost].
Although I notified you immediately of the loss of the card, you confirmed that the card had already been used to make the following purchase without my authority:
[Details of the transaction]
I would be grateful if you would credit the sum in dispute back to me and provide your confirmation that this has been done.
I look forward to hearing from you by return.