Limitation periods are laid down in the Limitation Act 1980. The expiry of a limitation period provides a defendant with a complete defence to a claim. The primary purpose of the limitation period is to protect the defendant from having to face a claim which he never expected having to deal with. Furthermore, if a claim if brought long after the events in question, it's very likely that some evidence may have been lost and memories of any witnesses will have faded. Once the limitation period has expired, a debt becomes statute barred.
5 Time limit for actions founded on simple contract.
An action founded on simple contract shall not be brought after the expiration of six years from the date on which the cause of action accrued.
Actions to recover money secured by a mortgage or charge or to recover proceeds of the sale of land
20 Time limit for actions to recover money secured by a mortgage or charge or to recover proceeds of the sale of land.
(1)No action shall be brought to recover—
(a)any principal sum of money secured by a mortgage or other charge on property (whether real or personal); or
(b)proceeds of the sale of land;
after the expiration of twelve years from the date on which the right to receive the money accrued.