Could do with a bit more clarity here.
The debt is over six years old, yes? But when was the last contact in either direction?
If there has been no correspondence for more than 6 years, a simple "Statute Barred Letter Template" from any number of websites will suffice. The new owner of the debt is unlikely to bother pursuing it further.
However, sending such a letter is a mistake if there has been any correspondence in the past 6 years. "Statute Barred" wouldn't automatically apply, and sending such a letter would simply be admittance of the outstanding debt. Instead, seek to test their confidence in the data they've inherited. Start off by stating that you have no record of the outstanding debt, and that you cannot find any record with any credit referencing agencies. See how they respond. Unless they're confident that they have the paperwork to back up their claim, they won't push far. Whatever you do, do not (at this stage) admit knowledge of the outstanding debt.
For anyone considering this underhanded, just bear in mind that the OP could see his credit rating trashed following any admission of the debt. Given the OP made some effort to settle the debt, that would seem unfair.
If their confident that they have a case then you probably need to seek professional legal advice. Chances are, the case will wind up in court.