A medical practitioner may, notwithstanding these Regulations, on the written request of a person in charge of, or the master of a ship or the officer in charge of an aircraft, authorise the purchase, acquisition, keeping or use of a Schedule 0, Schedule 1, Schedule 2, Schedule 3, Schedule 4, Schedule 5 or Schedule 6 substance: Provided that the quantity shall be reasonable and on condition that such medicine is intended only for emergency medicinal use on board such ship or aircraft.