This document is addressed to the registered car owner.
At the risk of seeming pedantic, there is no such thing as a 'registered owner', the DVLA keeps no such database, hence me asking if the document was addressed to the Registered Keeper. It's a distinction worth making as sometimes the owner may not be the registered keeper.
However, assuming you were using the terms interchangeably and the notice is addressed to the Registered Keeper, then they should appeal, in their capacity as keeper, with something such as the below:
Dear Sirs,
I have received your Parking Charge Notice (Ref: ________) for vehicle registration mark ____ ___, in which you allege that the driver is liable to pay a parking charge. I note from your correspondence that you are not seeking to hold me liable as the registered keeper, under Schedule 4 of the Protection of Freedoms Act 2012 ("The Act").
There is no obligation for me to name the driver and I will not be doing so. I am therefore unable to help you further with this matter, and look forward to your confirmation that the charge has been cancelled. If you choose to decline this appeal, you must issue a POPLA code.
Yours,
If appealing online, be careful of any drop down boxes etc. - it is important you do not reveal to APCOA who was driving. Keep an eye on your spam folder for their response in case it ends up in there. If you do not hear back from them within 35 days, chase them.