Please let us know if the known keeper identified as the unknown driver in the appeal?
Horizon rarely, and definitely in this case, issue NtKs that rely on PoFA to hold the known keeper liable. The known keeper can only be liable if Horizon fully comply with all the requirements of PoFA. As they haven't, only the unknown driver is liable.
Have you, the known keeper identified the unknown driver, inadvertently or otherwise? Once the driver is known, the "golden ticket" becomes a "silver bullet" for Horizon and getting the PCN cancelled become much harder.
So, has the driver been identified and thus become known?
Irrespective of signage and anything else argued, this could have been cancelled with the following simple appeal:
This is an appeal by the registered keeper - No driver details will be given. Please do NOT try the trick of asking for driver details in order to get around the fact your NtK does not comply with PoFA. As there is no keeper liability, therefore, liability cannot flow from the driver to the keeper and so, is an automatic win at POPLA. Please cancel the notice or issue a POPLA code at which point you will auto withdraw.