They are not relying on PoFA to transfer liability to the registered keeper and they have no idea who the driver was.
Hopefully an easy one to get past but it may take two appeal stages.
Firstly, it appears that you are involved with the church concerned so have you tried 'Route One', namely; get them to cancel it?
In the alternative, send the following to Horizon as an appeal made as Registered Keeper;
To whom it may concern,
I am the Registered Keeper of the vehicle in question.
I note from your speculative invoice that you are not seeking to rely on PoFA keeper liability.
So, as I understand it, you do not currently know the identity of the driver and, additionally, I have no legal obligation to provide an unregulated parking operator with those details.
I would point out that I understand that full payment was made in this instance.
Therefore, the options moving forward are;
i) Cancel the parking invoice
OR
ii) Unnecessarily waste further time, money and resources by providing me with the POPLA details and code etc.
Unfortunately, I am unable to help you further.
Best wishes,
xxxxxx xxxxxxx