We had a similar thing last year in St George's Retail Park in Leicester. It turns out the limit is 3 hours and we went over. The signs are sporadic at best, certainly not prominent and only readable if you walk up and take a close look at what it says! In my opinion these sign should be a LOT BIGGER and also a notice by the entrance to every shop!
To make matters worse, this is a retail park, with shops such as Mothercare, Toys-r-Us, a supermarket, McD's, KFC and more. We had a baby due, and a 4 year old in tow so had a lot to buy for those, and these decisions are not quick. The 4 year old got hungry so we had to go to KFC - and 4 year old's don't finish quickly! Finally, we had to get a mobile phone contract also in the same retail park and the usual story of a Saturday afternoon where we had to wait for the credit check for over an hour, by which time the assistant realised that he should have clicked "refresh" in the first place as it was already approved.
Long story short, we got an invoice in the post, we tried to explain to the company but they wanted bank statements, receipts, DNA evidence, witness statements from 3 proven virgins from different provinces and the promise of our first born son before they would even consider dropping the charge.
Luckily, the guys in the phone shop remembered us, fired off a quick email and we never heard anything of it.
So, they make a retail park with all the types of shops that can easily take all day and then put up tiny signs and issue "fines" after 3 hours, seems to be the latest money making scam under the guise of preventing abuse of the car park, but at the expense of innocent shoppers.