As Rich says.
You would have entered into an agreement with the parking operator by parking there. But the oOperators often fall down by not following the statutory specification (ie the law) which is detailed here
http://www.legislation.gov.uk/ukpga/2012/9/schedule/4/enacted
For example, i got a Notice to Keeper issues to me, i overstayed in MacDonalds once (work Meeting, why in MacDonalds is another story). I quoted the law to them and said that the Notice to Keeper was sent 1 day late and therefore fell out of the mandatory specification which they must adhere to (the law).
I also said i wouldn’t appeal it, i would absolutely refuse to pay it and i would not enter into any further correspondence, and any correspondence received by them or companies acting on their behalf may be considered as harassment.
Ive not heard a thing in 2 months.