Can you confirm that you are the registered keeper of the vehicle and hold the vehicle's V5C document in your own name? For interest (to check they are not misrepresenting their position) can you please show us the back page of the notice?
Assuming that is the case, CP Plus have issued the notice too late to hold the registered keeper liable, as they have failed to deliver the notice within the relevant period required by Schedule 4 of PoFA. You can therefore appeal online as the keeper with 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.