In whose name is the NtK? Is it in a company name?
The NtK fails PoFA 9(2)(e)(i) in that there is no "invitation" not any synonym of that word, for the keeper to pay the charge and is therefore not fully compliant with all the requirements of PoFA and so, the keeper cannot be liable. Partial or even substantial compliance is not sufficient.
For now, does the keeper know who the landowner is? Can they try and ask the landowner to get their agent to cancel the PCN.