Their notice makes no attempt to comply with Schedule 4 of the Protection of Freedoms Act 2012 (there's a link to it in my signature at the bottom of this post). Something along the lines of the below could be suitable...
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. 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 sure not to reveal who was driving, or tick any boxes confirming the same -
you're appealing as the registered keeper. Keep an eye on your spam folder for their response.