mccfluff has provided the correct link but I seriously suggest you take some advice from these guys:
http://www.consumeractiongroup.co.uk/forum/forumdisplay.php?167-Debt-Collection-Agencies
Type "Bryan Carter" into the CAG search box at the top of the page and then join the forum. Bryan Carter are well-known to operate on the margins of what are legal and the guys on the forum will help you through challenging their communications be it by phone or letter. Never be intimidated by these idiots. They are well known to send standard "threatograms" which have no legal standing in law.
if the debt relates to a credit card, they have to prove that they are entitled to collect via a copy of the original credit agreement. If they cannot, they can take a hike, with the blessing of the law. Never lose any sleep over these idiots as they often work on behalf of dubious debt collection agencies who are invariably working via threats etc.
I suggest you join the forum. The guys will help you through any threats received.