You're in luck - they're not seeking to hold you liable as the keeper, using the provisions of the Protection of Freedoms Act. Therefore only the driver is liable, and they don't know who that is.
You could appeal along the lines of the following, if you are the registered keeper of the vehicle (if you aren't, tell us):
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 compliant 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, you're appealing as the keeper, and keep a close eye on your spam folder for their response. If they do not respond within 35 days, chase them.