Respectfully, whilst
@H C Andersen is a trained lawyer and can apply his expert legal knowledge, in laymen's terms, you have been issued three "Parking" Charge Notices.
How do we know this? Because it says so on the reminders and from research on the InterWeb, it is known that Cambridge North car park is owned by the train operating company (TOC) Greater Anglia (GA). The land where the car park is located is a railway asset and covered by railway bylaws. NCP is contracted by GA to operate and manage the car park. NCP has stated, on record elsewhere, that they enforce the terms and conditions at the car park under contract law, not railway bylaw and they do not and cannot rely on PoFA.
Based on the above, NCP issue "Parking" Charge Notices not "Penalty" Charge Notices. NCP do not and cannot rely on PoFA to hold the keeper liable. The only way the keeper can be liable is if the keeper admits to also being the driver. There is no obligation in civil law for the keeper to identify the driver and no inference can be made.
NCP do not know the identity of the driver unless they have been told who it was by the keeper. If the keeper refuses the "invitation" to name the driver, NCP have nowhere else to turn.
Where this could become interesting, is if NCP insist on pursuing the keeper after their refusal of the invitation to identify the driver. In a simple, initial appeal as the keeper, refusing their invitation to identify the driver, NCP "should" throw in the towel. However, they don't tend to do so and hope that the keeper is low hanging fruit on the gullible tree and will eventually capitulate and pay up when things escalate.
This is when your initial appeal is rejected and you receive a POPLA code (which NCP pay £30+ for) and you appeal to POPLA, a supposedly "independent" appeal service, where an assessor will review your appeal. The assessors are not lawyers but do have some legal training. To what standard, we don't know. They will only assess whether the points you raise are enough to invalidate the issue of the PCN on the basis of compliance with the BPA CoP and legalities such as compliance with PoFA. Unfortunately, there have been quite a few instances where the assessor has misunderstood how to apply PoFA and retraining has been necessary.
In theory, with the extra training provided to the assessors in understanding PoFA, they now (mostly) know that a NtK issued for parking on land covered by bylaws, the PPC cannot hold the keeper liable for the charge. Only the driver can be liable and if the keeper has declined the invitation to identify the driver, the assessor will find in favour of the appellant and that is then the end of the matter.
In the now, somewhat rare instance, where an assessor gets it wrong regarding PoFA and denies the appeal, it does not really matter. You can complain to POPLA and they can fully agree that their assessor got it wrong but they cannot reverse a the decision. Not to worry... a POPLA decision is not binding on the appellant and would have no bearing on any future court claim should the PPC be reckless enough to pursue it that far.
In this instance, the victim would have to weather some months of useless debt collector letters. These debt collectors work on a no-win no-fee basis and have no power to do anything, no matter how scary they make their correspondence sound. As already mentioned, they are looking for the low hanging fruit on the gullible tree who will now become scared at the added £60-£70 to the original £100 charge and all the mention of "CCJs" and "bailiffs" and capitulate.
IF the PPC are really intellectually malnourished, they may engage (or do it themselves) a firm of roboclaim solicitors who will send the victim a Letter of Claim (LoC) giving the potential defendant 30 days to pay or face the possibility of a claim without further notice. If things ever got this far (and they do with a few seriously intellectually malnourished PPCs and their roboclaim solicitors) you
DO want them to issue a claim. A robustly defended claim highlighting the problem with the defendant not being liable as the keeper, will be an important fact that a district judge must take into consideration.
Thankfully, only a few intellectually malnourished roboclaim firms press on this far and even then, they tend to discontinue before it ever gets in front of a judge.