Firstly it can’t legally be called a “fine”, so if that word is written on the notice then the notice itself is illegal. If it is called a “parking charge notice”, it is legal and if this is a private parking area and a condition of parking there is that you shop in the shops that own it then you are liable for depriving the shop of a customer parking space.

Whether you are liable for £55 is another matter and the parking enforcer would have to take you to court for that. In this circumstance my view is you should send the parking enforcement agency £10 in "reasonable settlement for my breach of parking contract of which I was unaware due to the inadequacy of the signage." Take a photo of the sign close-up and from about 15 feet away in case you need to illustrate your point to a court.

Of 1,800,000 private parking tickets issued last year, only 845 went to court proceedings, only 49 actually to court and only 24 were found in favour of the plaintiff.