18.—(1) Where no appeal is brought against the decision to impose a variable monetary penalty, the penalty must be paid within the period of 56 days beginning with the day on which the final notice is served, or such later period as the enforcement authority may agree in writing.
(2) Where an appeal is brought but a variable monetary penalty remains payable following that appeal, the penalty must be paid within the period of 28 days beginning with the day the appeal is determined.