We have a private customer only carpark in the rear of our shop that is being frequented by non-customers. We are yet to do anything about it but was given advice that if we were to follow this sort of signage we can take action. The advice is as follows: "if there is a clear sign outside the car park entrance that warns drivers if they do not have permission to park, they consent to their cars being clamped or towed, and the sign is enforceable as a "contract" then they will have effectively given you their consent." Is this advice sound?