The Notice to Keeper is PoFA compliant which means that if the driver is not identified, they can hold the Keeper liable for the charge. The only way they cannot hold the Keeper liable is if the signs are incapable of forming a contract because the charge is not adequately brought to the attention of the driver. PoFA 2(3)(b)(ii) applies.
You would need to show us a close up of the sign and then a photo of the sign in the same lighting conditions at the time of the alleged contravention. Do not use flash or headlights. Also a general view of the car park in the same lighting conditions. Does the car park have an entrance sign that warns the driver that it is private land and that they should seek the sings with the terms and conditions?
Al these are questions that need answering and are useful in a defence should this ever escalate to litigation.