You don't say who your original debt was with but it sounds like they have sold the debt on to a debt collection agency.
Firstly - do you owe this money?
If you do, then the bad news is that old debts never go away. You mustn't bury your head in the sand and ignore it.
I think your best bet is to contact your local Citizens Advice Bureau. They will have a member of staff dedicated to debt problems who will act as go-between, and will know the ropes and terminology. If you owe, it, you'll have to pay it, but the CAB will be able to negotiate a repayment plan that hopefully, you can afford, even if it's only a few pounds each week.
DO NOT use any of the so called debt management firms that advertise everywhere as they'll cost you an arm and a leg.
Good luck.