You should have responded with a simple appeal telling APCOA that you are the registered keeper and you are under no obligation identify the driver. As the location is covered by byelaws, it is not relevant land for the purposes of PoFA and so there can be no Keeper liability. They should therefore chase the driver, whoever that may be.
That would have been the end of the matter. However, by ignoring that, you are now in debt recovery mode. You ca safely ignore any debt collector letters and no one needs to know about them. 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.
APCOA will not litigate and so, all you can do now is continue to ignore all the debt collector letters until they give up and go away.
