Everything you suggest is mere mitigation, and unfortunately at this stage we need something a bit more legalistic.
Dear Plymouth City Council,
I challenge liability for PCN PL10903622 on the ground that the alleged contravention did not occur. Firstly you allege that parking may be paid for via a cashless system, but upon consulting The City of Plymouth (Off-Street Parking Places) Order 2007 as amended by The City of Plymouth (Off-Street Parking Places) Amendment Order No. 14) 2017 and The City of Plymouth (Off-Street Parking Places) Notice of Variation (No.1) 2023, the order does not provide any cashless payment option at all.
Furthermore, the contravention alleged on the PCN is "Parked without a valid virtual permit or clearly displaying a valid physical permit where required", but it is not physically possible to display a virtual permit. Even if a valid virtual permit is in force, this will not be displayed. The allegation stated on the PCN is essentially meaningless.
I further allege a procedural impropriety because the penalty charge notices does not convey the requirements of regulation 3(1)(c) of The Civil Enforcement of Road Traffic Contraventions (Representations and Appeals) (England) Regulations 2022.
If you do not accept any of the above, I nonetheless invite you to reconsider whether it would serve any traffic management purpose to pursue enforcement, considering that parking was paid for in any event.
Yours faithfully,
Give it a couple of days to see if anyone has any other suggestions. Send the representations online and make sure to keep a screenshot of the confirmation screen.