So, tell them to send a formal complaint to Horizon about not being able to appeal (or pay) and refer them to the answer given in Arkell v Pressdram (1971).
They are obliged to consider a complaint as an appeal. considering that most Horizon PCNs are not PoFA compliant and therefore the Keeper cannot be liable and they have absolutely no idea who the driver is, unless the Keeper blabs it to them when there is no legal obligation to do so.
So, paying a Horizon PCN when there is no legal reason to do so, makes the recipient of the PCN a fool and their money.
Can you show us the Notice to Keeper (NtK) that was received and we can suggest a suitable formal complaint that will include the necessary wording to go tell them to chase the unknown (to them) driver?