If the debt has gone over the 6 year period and you have not had a CCJ issued against you then the debt is unenforceable and has become Statute Barred. The debt still exists but once it has become statute barred the creditor can no longer use the courts to collect the debt.. They are also no longer allowed to harrass you to collect the debt.
If the company has got a CCJ in the mean time then that is enforceable forever and can be chased forever. You need to make sure that it was 6 years since your last payment and they have not got a CCJ against you, if they havent then the debt is statute barred.