how long ago did you receive it as legally they have to take you to court within a time limit 101 & 366 days ring a bell in my head pending on what the case is criminal / compensation
About 18 months between 1st and last letters.
I've had this happen twice, the last time was overnight on a car park for a retail centre that also has apartments on one side.
Now who is going to use the retail centre overnight??
There were no signs saying overnight parking wasn't allowed, just a limit of 2 hours free parking between 8 and 8.
Basically they are trying to threaten you to pay.
A lot of people just panic and pay straight away as the half price offer seems a bargain.
If you don't have any contact at all they usually give up as it's not worth the effort to continue, they make enough from the people who do pay.
If you enter any dialogue with them it's seen as an admission that you had parked there.