The amendments specified in Schedule 6 to this Act shall have effect for the purpose of enabling a court-martial which imposes a fine on a person convicted of an offence under the [1955 c. 18.] Army Act 1955, the [1955 c. 19.] Air Force Act 1955 or the Naval Discipline Act 1957 to impose, in certain circumstances, a term of imprisonment not exceeding 12 months in default of payment of that fine.