A couple of questions to clarify:
- Is this the first notice you have received?
- Out of interest, can we see the back of the notice?
If this is the first correspondence you have received, it has been issued far too late for them to hold you liable as the keeper, using the provisions of Schedule 4 of the Protection of Freedoms Act (there's a link to it in my signature for some background reading on keeper liability). With that in mind, I'd submit a simple appeal 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 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,
Appeal only as the keeper - watch out for any tick boxes etc. on the online form that may seek to trick you into revealing who was driving.