Often you will see parking companies have a last try just before the 6th year anniversary of the parking event. After this length of time, it comes as a bit of a shock as people don't have any paperwork or recollection. This is where the Limitation Act 1980 comes into play
s5 of the Act says
"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."
In plain english, an action (court case) based on a simple contract (a parking ticket) cannot be brought (started) after the expiration of six years (anniversary date) from the date on which the cause of action accrued i.e. parking.
So after 6 years the issue dies but the debt collectors will continue to mislead before then and even harass you afterwords. If needs be, report them to Trading Standards or their regulatory bodies.