It is too late to appeal anything now. Do NOT try and contact APCOA to explain anything. As you can see in your post, you have already committed the cardinal sin of identifying who was driving.
You should edit your post to remove anything that gives away the identity of the driver. The driver and the keeper are two separate legal entities. Only the Keeper is "known" to APCOA. Only the driver can be liable unless they've complied fully with all the requirements of PoFA 2012.
You can safely ignore all debt collector letters. DRP and their ilk are powerless to do anything. You must never, ever, ever, communicate with a useless debt collector. They are not a party to any contract allegedly breached by the driver.
You need to find out why you did not receive anything before the DRP letters. The most common reason is that your V5C was not updated with your current address after your last move. Updating your drivers licence with the DVLA does not update the V5C. You can check online and update it there if necessary.
If your address was not uo to date on your V5C, you need to submit a Data Rectification Notice to the DPO at APCOA advising them of the correct address for service of documents and to erase your old address and to tell anyone they may have passed your details to, to do the same.
For now, continue to ignore any debt collector letters. APCOA are benign and do not tend to litigate. Eventually, they will stop and that will be it. However, in the extremely remote chance that you receive any other correspondence that is not a debt collector letter or reminder, let us know.
