I visited a B&Q car park a few days ago. The warning signs were reasonably conspicuous but I probably wouldn't have taken the trouble to read them if I hadn't been aware of this discussion. Apart from a time limit (which I think was 2 hours) there is also a rule about not returning within 4 hours.
A customer who complains about a charge days or weeks later and says "but I was shopping in the store the whole time" is not likely to get much sympathy, I shouldn't think. After all, that's what he's bound to say even if he went to a different store or to a football match nearby.