It says in the last sentence of the first paragraph the PCN has not been paid in full. That is incorrect. We didn't receive anything.
The driver was in the car all the time and hadnt seen any sign.
I don't quite understand how "we didn't receive anything" relates to "their PCN has not been paid in full". Can you please elaborate on that so my confusion can be corrected?
That aside, this is a defendable but will probably have to go as far as Plan C. Plan A, depending on your reasons for being at the location, have you tried to contact the landowner and get them to get this PCN cancelled?
In order to be able to progress to Plan C, you must do Plan B which is an appeal to Premier Park, which they no doubt will reject but that rejection gets you a POPLA code which is required to progress to Plan C.
Hopefully, a good POPLA appeal will be successful. However, if it isn't, never mind. A POPLA decision is not binding and you would then progress to Plan D which is have a judge decide whether you owe a debt to Premier Park.
Whilst the NtK is mostly PoFA compliant, there are technical issues with it that could be argued that invalidate it being able to hold you, the registered keeper liable for the charge. So, appeal only as the keeper, not the driver. Do not identify the driver with words like "I parked..." or "I drove..." etc.
They are claiming a brach of contract but there was no contract to be breached because the signage is prohibitive. In order for a contract to be agreed by conduct, there has to be an agreement. Such agreements MUST include:
An offer
Acceptance of that offer
Consideration - something of value must pass both ways and
Intention to create legally binding obligations.
If the signs are what is known as "forbidding", "No Parking" means they contain no offer - "you must not park here" is not an offer.
By stopping there, you were not agreeing to a contractual term, you were potentially trespassing. That is a completely different area of law. No sign can set out contractual terms for trespass.
So, appeal as the keeper and tell them that, according to the driver, there were no prominent or obvious signs visible from within the vehicle that offered any contract to park at the location for £100. You deny any liability as the registered keeper.
After the inevitable rejection, you move on to a POPLA appeal which will go much further into the legal and BPA Code of Practice details as well as technicalities that make the NtK non-PoFA compliant. POPLA does not consider mitigating circumstances, only whether the PCN/NtK has been issued correctly according to the BPA CoP and the law.
Show us what you propose to put in your Plan B appeal to Premier Park before submitting anything.