It's my opinion that, as any debt which you may have and is over 6yrs old, the claim for settlement is statute barred by the Limitation Act 1980.
Personally, I would just ignore the claim and not correspond any further with the Debt Collection Agency.
However, as I am not a Solicitor, perhaps one of AB's Legal Eagles will comment if you were to post in the Law section and draw attention to this thread and the personal opinions which I have given.
Ron.