You seem to be hell bent on attacking this your way. C'est la vie.
IMO, you are approaching the issue in the wrong manner. Set out below is an extract from the Secretary of State's Statutory Guidance to which ALL councils with enforcement authority powers must have regard:
An authority has a discretionary power to cancel a PCN at any point throughout the process. It can do this even when an undoubted contravention has occurred if the authority deems it to be appropriate in the circumstances of the case.
Under general principles of public law, authorities have a duty to act fairly and proportionately and are encouraged to exercise discretion sensibly and reasonably and with due regard to the public interest. Failure to act in accordance with the general principles of public law may lead to a claim for a decision to be judicially reviewed.
Enforcement authorities have a duty not to fetter their discretion, so should ensure that PNCs, NtOs, leaflets and any other advice they give do not mislead the public about what they may consider in the way of representations.
They should approach the exercise of discretion objectively and without regard to any financial interest in the penalty or decisions that may have been taken at an earlier stage in proceedings.
Authorities should formulate (with advice from their legal department) and then publish their policies on the exercise of discretion. They should apply these policies flexibly and judge each case on its merits. An enforcement authority should be ready to depart from its policies if the particular circumstances of the case warrant it.My emphasis.
IMO, get their policy by asking for it. Once you get it, you have the starting point for your investigation.
And note, the policy is the council's, NOT parking's. The officers employed* in this department are bound by the policy; it is not 'discretionary' whether they apply it or not.
And, as already suggested, get copies of their Annual Reports, maybe for the last 2-3 years, and see how their use of discretion has been reported.
*- which includes any contractors involved in the process of 'consideration of challenges and representations'.
Link to the Guidance:
https://www.gov.uk/government/publications/civil-enforcement-of-parking-contraventions/guidance-for-local-authorities-on-enforcing-parking-restrictions#training-and-professionalism-in-civil-parking-enforcementNote that there is a separate section on The Exercise of Discretion(which refers to on-street activities) and a clear reference to the requirement to comply with other legislative imperatives e.g. Equality Act etc, and their public law duty to act fairly and reasonably.