Appealing on the basis of the correct VRM should see a reduced charge of £20 offered for a 'Major Keying Error'. However, as the registered keeper, you have another option that should see you pay nothing. If they want to recover the charge from you as the keeper, using the provisions of Schedule 4 of the Protection of Freedoms Act 2012, they must deliver a notice to keeper within 14 days. The notice issued to you was issued on day 13, and on the basis that notices by post are presumed to be delivered 2 working days after the date on which they were issued, your notice arrived too late. You could therefore 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. 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,
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.