Time is always allowed for making payment or else nobody could ever park legally ! However, walking away from the car and making payment via mobile phone is never a good idea. You should submit representations on the basis that the mobile signal was poor so you had to move away from the car in order to make payment. Enclose a copy of the payment record. However, don't expect them to accept your reps, because councils reject virtually all informal challenges because they know that most people then just cough-up to get the discount.
