Whatever you do, do not reveal the identity of the driver. The location is not relevant land for the purposes of PoFA which means that only the driver can be liable for the charges. You, as the keeper of the vehicle are a separate entity in law and because the PCNs are not able to rely on PoFA, you cannot be liable for them.
You are probably too late at this stage to do anything except wait for them to send you a Letter of Claim (LoC) and eventually a court claim. You can safely ignore anything else they send to you such as reminders and anything form their chosen debt collectors. No matter how scary those letters may seem, they are safe to ignore. You must not ignore an LoC or court claim.
I believe there was an issue last year when NCP updated their app but failed to notify existing users that they needed to delete the original app and download a new version. Anyone using the old app after whatever cut off date they had, ended up receiving PCNs. Of course, they will not admit to this but it is a fact.
You may want to check the address on your V5C logbook to make sure your current address is shown correctly on it. If you moved at any time and forgot to update it, it could be one of the reasons you did not receive the original NtKs. Many people update their drivers licence but fail to update the V5C. Whilst both are handled by the DVLA, they are not connected in any way.
If the V5C address is not up to date, you must get it updated asap. However, you
must also
instruct the DPO at NCP to
rectify your data with your current address for service and for them to
erase the old address.
You should also try, in any case, an appeal to NCP (although they may refuse to accept one) as the keeper along these lines for each PCN:
I appeal as keeper. I am under no obligation to identify the driver and I decline to do so as there is no legal presumption that the keeper of a vehicle was its driver (as opposed, for example, to being a passenger) on any particular occasion, you are unable to pursue me as the driver.
As your allegation of a breach of terms occurred on land subject to statutory control by bylaws, it is not relevant land for the purposes of the Schedule 4 to the Protection of Freedoms Act 2012 (PoFA) and therefore there can be no keeper liability.