Aberdeen is blessed with three Icelands
Aberdeen is also blessed, like the rest of Scotland, with no current provision for Keeper Liability, in the way that it exists South of the border (for now, this may well change in the nearish future, so don't assume the same is true if you get any future charges). I assume you as the keeper also reside in Scotland?
If so, a simple appeal to Horizon along the lines of the below, as keeper (being very clear not to tick any boxes etc. revealing who was driving on the online form):
Dear Sirs,
I have received your notice (Ref #:_______) for vehicle registration mark _____. I am the registered keeper of the vehicle. There is no obligation for me to name the driver at the time and I will not be doing so.
As the Protection of Freedoms Act 2012 does not apply in Scotland, you are not able to recover the charge from me, the registered keeper, and I therefore have no liability in this matter.
I look forward to your confirmation that this charge has been cancelled.
Yours faithfully,
Horizon are one of the few that do often cancel charges on first appeal, even in England, so that will likely see an end to the matter, but let us know how you get on.