I agree. After many attempts over the years, Horizon recently started to do something about their shoddy PCNs and have finally honed them to PoFA perfection.
I blame myself and a few others who repeatedly, over the years, managed to have every single claim by Horizon thrown out, where the drivers identity was not revealed.
So, as far as this PCN is concerned, you won't win it on no Keeper liability, unless the location, which you've inconveniently redacted, introduces any ambiguity about the relevant land.
You'd be better trying to appeal on poor signage and the possibility that their contract with eh landowner or their agent is not valid.