OP, you need to get your head around what you think the "14 day rule" is.
The parking firm have no idea of the drivers identity unless the Keeper who has received the Notice, blabs it to them. Only the driver can be liable of the charge.
Where the "14 day rules" comes in, is if the parking firm want to try and hold the known Keeper liable for the breach of contract by the unknown (to them) driver. In order for them to be able to transfer the liability from the union driver to the known Keeper, they have to fully comply with ALL the requirements of PoFA 2012.
One of those requirements is that if they have issued the Parking Charge Notice (PCN) as a postal Notice to Keeper (NtK), as in this case, that NtK must be delivered (given) to the Keeper within 14 days of the alleged contravention. They are required to allow two working days for delivery from the date the NtK is issued.
If the NtK was issued too late for it to have been given (deemed, not actual), then that is one of the requirement of PoFA they have failed to comply with and so they cannot rely on PoFA to be able to hold the known Keeper liable for the alleged debt owed by the unknown driver.
However, there are many more PoFA requirements they need to comply with and the "14 day rule" is just one of them. In this case, they have failed others too.
A PCN cannot just be mostly or even substantially compliant with PoFA. It is a binary matter. It either is or it isn't. Just like someone can't be mostly or even substantially pregnant. They either are or they aren't. Simples.