First things first, the notice you have uploaded appears to be addressed to a company, rather than you as an individual. Did your hire company forward it on rather than nominating you as the hirer like they should have done? Second, what did you say in your appeal, did you reveal who was driving?
We could do with seeing some photos of the car park signage, as well as the signage around the EV charging points. We're seeing this happen fairly often, where an EV charger provider is happy for their chargers to be used for hours, but the car park they put those chargers in only allows parking for a much shorter period.
Citizens Advice suggests either (1) paying under protest and then filing a claim to recoup
This wouldn't be my recommended course of action. This isn't a situation where you're facing some sort of compulsion to pay (like in the old days of clamping where you had to pay if you wanted to drive your car away). Given there's no compulsion to pay, the first question that would arise if seeking to recoup would be: why did you choose to pay if you didn't owe the money?
I understand the worry about rising fees via debt collectors, but two things to consider there. First, debt collectors cannot force you to pay, only a court can. Second, the fees that they add can usually be successfully challenged in court, even if you lose (the £100 parking charge should include the costs of debt recovery).
Since I appealed, I now also note that the original enforcement notice seems to have an error. They state that their T&Cs are for a restricted time period between 2am and 10am but that my stay was from 3.29pm and 6.57pm. I think they have their time shown the wrong way round but can I still challenge on this basis?
They seem to have included a bit of a 'catch all' term there, saying "
Exceeding the notified maximum free parking period of 90 minutes / remaining in the car park during notified restricted hours (2:00AM to 10:00AM daily)". Your charge would seem to have been issued for the reason highlighted in bold here.
Aside from any formal appeals and court actions etc., you should seek to contact the owner/managing agent of the car park, to see if they will intervene. If you're struggling with them, see if the electric charger company can help you contact them.