Soounds like a scam to me,, there was a very similar posting a few weeks back.. Up to them to prove you owe it, I believe - if it goes to course and you have never owed this money, they will look pretty silly. However debt collectors can only be called off by the company who instructs them, so it is worth your sending a recorded delivery letter to the company saying that you don't owe this money (presuming that you don't, of course) and can they stop pestering you.
Btw is the letter personally and correctly addressed to you - not just to the Occupier or anything vague like that?
Don't be taken in by the threats .