Horizon still don't bother to use Schedule 4 of the Protection of Freedoms Act 2012 to hold the keeper liable. So appeal as the registered keeper along 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 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,
Let us know how you get on - they usually cancel. Keep an eye on your spam emails if appealing online, and if you don't get a result within 35 days chase them.