If you are prepared to stand your ground on this, you have a good chance of winning at London Tribunals, because the PCN says you didn't pay the parking charge, but there is no parking charge for the time you were there.
So I suggest you submit representations that you made every attempt to register a free parking charge, but The Ringo app, mandated for this car park by the council, would not accept your input for many minutes, but you did finally manage to register your car. tell them that in between your time of arrival and the registration time, a CEO served a PCN. You request that the PCN be cancelled as you made every attempt to register but this took several attempts. Don't be upset if they refuse these reps, because as far as councils are concerned, PCNs are there as their Magic Money Tree, and they know that if they refuse all informal reps, most people, (like >95%) just cough-up to get the discount. The basis of paid parking is that time is always allowed to pay for parking, and your case is the same.
