You do not assert that MET do not have a valid contract flowing from the landowner. You don't know that. You DO put them to strict proof that they have a valid contract.
Unfortunately, POPLA assessors are pretty thick when it comes to proof of a valid contract because they will accept a simple statement that one exists as "proof", together with the absurd argument that if the landowner has allowed signs to put in place, then they must surely be permitted.
You have to explain to the POPLA assessor that any contractual proof will have to fulfil what is required in the PPSCoP. So, whatever that provide as their evidence of a valid contract, it must show what is required in section 14 of the PPSCoP:
14 Relationship with landowner
14.1. Where controlled land is being managed on behalf of a landowner(s), before a parking charge can be issued written confirmation must be obtained by the parking operator from the landowner(s) covering:
a) the identity of the landowner(s);
b) a boundary map of the land to be managed;
c) such byelaws as may apply to the land relating to the management of parking;
d) the permission granted to the parking operator by the landowner(s) and the duration of that permission;
e) the parking terms and conditions that are to be applied by the parking operator, including as appropriate the duration of free parking permitted, parking tariffs, and specific permissions and exemptions, e.g. for staff, residents or those stopping for short periods such as taxi and minicab drivers, delivery drivers and couriers;
f) the means by which parking charges will be issued;
NOTE 1: For example, to the windscreen or through the post.
g) responsibility for obtaining relevant consents e.g. planning or advertising consents relating to signs;
h) the obligations under which the parking operator is working, in compliance with this Code and as a member of an ATA;
i) notification of the documentation that the parking operator may be required to supply on request to authorised bodies detailing the relationship with the landowner; and
j) the parking operator’s approach to the handling of appeals against parking charges.
NOTE 2: Where byelaws have been made, which prohibit the issuance of a parking charge, unless specific legal provision has been made to suspend them, they take precedence and therefore careful consideration must be given to ensuring that the parking management arrangements are consistent with them.
NOTE 3: Particular care is needed to establish appropriate contractual terms, including the application of parking terms and conditions, in respect of controlled land where leaseholders may have rights that cannot be qualified or overruled e.g. by imposing a requirement on the resident of an apartment block to display a permit to park in contravention of their rights under their lease, or to ensure that free parking periods do not breach planning consents.
So, you have to lead the POPLA assessor by the nose that a simple signed statement is not evidence, especially if the signatories to the statement are not identified by name and their standing to sign on behalf of their companies. Good luck with getting that agreed to, but it doesn't hurt to try.