When you get the ticket it has the magic word PCN on it, now for the difference, PCN has 2 wordings, if it is Penalty Charge Notice then you will have to pay as this is enforceable, usually council or Police issued. Now for the other worthless piece of paper, It will say Parking Charge Notice and there is the difference. Because it is a free car park what can they actually claim a loss on for you parking there ? this is where court cases trying to enforce this has failed and they know this.
Parking eye who do B&Q will send you the so called fine offering a redution if you pay early, after this a picture of your car entering and leaving will be sent(picture wouldn`t identify owner in a court of law), next up is the threatening letter with a nice official police looking enforcement threat than usually last warning in red lettering (this really scares you) 12 months later and not a peep so don`t be bullied into paying up. I`m sure LL could add to or amend if I got anything wrong.