The first thing to do is to check with the pub. The parking charge is probably for people parking there and not using the pub. If the landlord is uncooperative (unlikely) then you have to go through an appeals process, and, if both stages of appeal fail, challenge the enforcement agency to take you to court. In the extremely unlikely event of that happening, get some photos of the notices in the car park to prove that they were not big enough to see and make sure you have a receipt to prove that you are in the pub. More here: www.honestjohn.co.uk/faq/private-parking-penalties/