I believe it is now, but at the time (few years ago) was fully legal as I did check quite thoroughly.
Somewhere in the car park will be a sign with the terms and conditions that say something like parking for customers only, max-2-hours etc. I was not a customer, had left the site and was therefore in breach of the terms & conditions which were enforced by an external company (Thug & Co Ltd) employed by the landowner.