Debt companies have to provide evidence of the debt, so as maxsteam advises, just tell them that you are happy to go to court, as so far they have not provided any evidence that the amount is due. Ask them to supply evidence from 02 as you have not received any letters from them advising that money is outstanding.
If you are sure the 02 debt is wrong, enter into a complaint with 02. asking them to furnish you with statements to show why the amount is due. Thia should confirm the date of the last payment and if over 6 years has elapsed, the debt may be statute barred and therefore unenforceable in court.
Check your credit records with Experian and Equifax to see if this is noted, so you can get the records corrected if wrong. Both these credit references companies offer free trials if you remember to cancel.