Are you saying that it has only just been received? Was it delivered to an address you no longer reside at and only now passed on to you? Is your V5C address up to date?
Too late to appeal now. It would have been an easy one to deal with at the time... as long as the unknown drivers identity is not revealed. There is no legal obligation on the known keeper (the recipient of the Notice to Keeper (NtK)) to reveal the identity of the unknown driver and no inference or assumptions can be made.
The NtK is not compliant with all the requirements of PoFA which means that if the unknown driver is not identified, they cannot transfer liability for the charge from the unknown driver to the known keeper.
As there is nothing that can be done, unless you can explain why it has only now been received, you should ignore all debt collection letters. They are harmless and must be ignored. You will have to wait and see if they try to issue a claim for the alleged debt.
If you have an opportunity, you can get a phot of the signs. Sometimes, ParkMaven and a sister, but separate company, Bransby Wilson, have signs that have the other company's logo on which invalidates the PCN.