Ignore the useless DRP. I wouldn't worry about it, although it was a silly mistake. For any future PCN, you never, ever, EVER communicate with a uselsss debt recovery agent. They are not a party to any contract the driver allegedly breached and are powerless to do anything except to try and intimidate the low-hanging fruit on the gullible tree into paying up out of ignorance and fear.
As this is APCOA, nothing will ever come of it, even is you do nothing except ignore the debt recovery letters. APCOA never litigate and you are unlikely to be the first. They make more than enough money from the low-hanging fruit.
You may want to ask why no initial Notice to Keeper (NtK) or reminder was ever received. In the vast majority of cases it is because when you last changed address, you may have updated your drivers licence but failed to update the V5C which is done separately. Can you confirm whether the V5C is up to date?
