So where were you when the PCN was issued ? Whilst time is allowed to pay for parking, it is never a good idea when using PbP to not be at the car-park. A CEO arrives, checks the cars for PbP parking that have no machine ticket, and if no session, issues a PCN, simple, what else should he do ? However, your narrative indicates you were in the process of paying for the parking, but took a bit longer than normal. So you need to submit representations that you were engaged in paying for the parking using your phone when the CEO issued the PCN. If they refuse your reps you can take them to adjudication on the matter, but the full PCN penalty will be in play.