The grace period point is probably not relevant here - the allegation from APCOA isn't that the driver overstayed, but that they didn't pay. It's clear from your account that they did, albeit whilst making a minor keying error when inputting the vehicle reg. My view would be to make an appeal as the keeper (being careful not to identify who was driving), along these lines, providing a copy of the ticket if their system allows.
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. The driver purchased a ticket that covered the full duration of their stay, a copy of which is attached. The driver made a minor keying error inputting the VRM, inputting ____ ___, instead of the correct VRM ____ ___. This falls within the scope of a minor keying error, as specified by 17.4 of the BPA's Code of Practice - the charge should therefore be cancelled.
In addition, 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"). As this is a railway car park covered by byelaws, it is not relevant land for the purposes of the Act.
There is no obligation for me to name the driver and I will not be doing so. For the reasons outlined above, I 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 35 days, chase them.