This is a case to do with warning motorists about the possibility of being clamped, or in today's terms, being given a parking charge. While this was a win for the motorist, parking companies often quote it as justification. This is based on the comments
"2387 The recorder found that the defendants had been entitled to clamp the plaintiff's car. The recorder went on to find that the charge that the plaintiff had had to pay was not exorbitant. The recorder also found that the matter should either have been settled or have gone to arbitration"
Wikipedia's summary says
"In summary, the decision established that applying a wheel clamp to a vehicle constitutes a trespass to goods and that the onus remains with the clamper to demonstrate that the person parking the vehicle knew of the risks and happily took these on at the time that he parked the vehicle. Although it might reasonably be inferred that a motorist saw and understood the signs as a result of their numbers, size and location it was insufficient that an appellant had simply had the opportunity to see warning signs but that they must also have read and understood them and only then, by doing so, could they consent to the act of clamping if they parked in contravention to the notices."
You'll notice that many parking company court claims refer to number, size and location of the signs. Size isn't everything so check the signs were there and were readable.