Sounds like a civil debt for which they will have to apply to a small claims court and demonstrate that you owe it to them.
OR… it may well be Blackmail – section 21(1) of the Theft Act states:
"A person is guilty of Blackmail if, with a view to gain for himself or another or with intent to cause loss to another, he makes any unwarranted demand with menaces; and for this purpose a demand with menaces is unwarranted unless the person making it does so in the belief:
a) That he has reasonable grounds for making the demand; and
b) that the use of the menaces is a proper means of reinforcing the demand."
The word Menaces in this context has been held to be a threat (including veiled one) of any action detrimental or unpleasant to the person addressed or something which would intimidate or influence the response.