As long as the signs were clearly displayed, you entered into a contract to observe the conditions on them when you parked your car. Contrary to what some websites will tell you, that contract is legally enforceable:
http://www.rochdaleon...alty-in-landmark-case
In Stowmarket, Lidl (or the car parking company acting as their agents) had people watching customers who left the store, to see whether they went straight back to their cars or whether they went to do some shopping elsewhere. Those who left the Lidl site received penalty notices (because it was a breach of the contract which they'd entered into), even if their car was only on the Lidl car park for less than the time which would have been allowed if they'd been shopping in the store.
Chris