A couple of things to clarify before anyone rushes in to appeal... In your opening post you say "we" have received a PCN. Unless the PCN is addressed to a company, there is no "we" - the notice is addressed to one person, and it is they who must respond.
Secondly, is the person to whom the PCN is addressed the registered keeper of the vehicle? By that I mean they have the car's V5C document in their possession with their details on - not a lease/finance car etc.
If so, then that person can appeal with the below:
Dear Sirs,
I have received your Parking Charge Notice (Ref: ________) for vehicle registration mark ____ ___, in which you allege that the driver has incurred 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"). You have chosen not to issue a Notice to Keeper in accordance with The Act, and it is now too late for you to do so.
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 there are no drop down/tick boxes that cause you to identify who was driving - select registered keeper only, and keep a close eye on your spam folder for their response. If they do not respond within 28 days, chase them.