Unusually, they seem to have calculated the PoFA period wrong - their notice says:
You are advised that, if after the period of 30 days beginning with the day after that on which this Parking Charge Notice to Keeper is given...
Paragpragh 9(2)(f) of Schedule 4 the Protection of Freedoms Act says the notice must:
warn the keeper that if, after the period of 28 days beginning with the day after that on which the notice is given...
.
On that basis, I'd be tempted to appeal as the keeper on the lines of 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. You are unable to hold me liable as the registered keeper, under Schedule 4 of the Protection of Freedoms Act 2012 ("The Act"). You have failed to issue a Notice to Keeper properly compliant 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, and keep a close eye on your spam folder for their response. If they do not respond within 28 days, chase them.