Quite a lot of these old debts are creeping out of the woodwork at present. I think a fair number were old one sold on by various banks. The vast majority are statute barred because no payment or acknowledgement has been made in the last 6 years, but there are some exceptions to this rule.
Also, the firms chasing the debts often have no certain knowledge that the person they are chasing is actually the debtor so it is quite likely you do not owe it.
Do not admit liability & do not pay anything. Ask for proof that you do owe this money. If you look around some other sites - Comsumer Action Group; or www.debthelpuk.co.uk you will find standard letters to modify and send with the necessary fee (�1 I think).