+1
Too many people walk away from the car when using a PbP app to pay for their parking, so a CEO turns up, sees no parking session on his HH terminal so serves a PCN. Meanwhile the session is set-up whist he's doing this. From what you have said you have a reasonable hope of success at London Tribunals.I say this because all London councils are now getting such huge revenues for parking penalties, (like over £700 million !!), they now just refuse ALL reps knowing most people, (like >95%), then just cough-up not knowing the law and fearful they'll get walloped for undreds of pound. Also not everybody knows that there is this forum to get advice from. So the only place you'll get a reasonably unbiased judgement on your case is London Tribunals.
Looking at your timings, I think you have a reasonable case, but there is no guarantee you'll win at LT. Best thing to do now is to submit representations on the basis that you were engaged in paying to park when the CEO served the PCN. State that you had to move away from the car to get a good signal and say that the fact that the parking commenced only one minute after the PCN was served shows that you were engaged in making the payment at the time the CEO served the PCN.