I'm assuming that you made the cardinal mistake of identifying the driver. As the Keeper, there is no legal obligation on the known Keeper to identify the unknown (to APCOA) driver. Their Parking Charge Notice (PCN) is not compliant with PoFA 2012 and so cannot hold the known Keeper liable, only the unknown (unless you blabbed it) driver.
A simple appeal, only as the Keeper, declining to identify the driver would have killed this off either at the first appeal or certainly at a secondary appeal to POPLA. Too late to do that now.
Luckily for you, APCOA is benign and does not litigate. As for the debt recovery letters, you can safely ignore those. Debt collectors are powerless to do anything except to try and persuade the low-hanging fruit on the gullible tree to pay up out of ignorance and fear.
If you continue to ignore all the debt collector letters, they will eventually stop and APCOAS will write it off. In the incredibly unlikely event that you ever receive a Letter of Claim (LoC), come back and tell us.
