The 6 years runs from the 'Cause of Action' and the Terms and Conditions of the contract/agreement will have a bearing on this.
Once a debt has lapsed, it cannot be revived - even through a subsequent acknowledgement or payment.
The relevant law is contained within the Limitation Act 1980.
In Scotland a debt generally becomes unenforceable after 5 years.
Also read the new CONC rules from the FCA - especially
Conc rules on statute barred debts which make powerful new provisions in respect of the pursuit of statute barred debts.
In respect of unlawful charges claims or PPI claims, you have 6 years to claim from the date on which you could reasonably have discovered that you should not have been paying the money.