(4) Any question arising under this paragraph as to what reduction is appropriate in the compensation payable in respect of the compulsory acquisition of an interest in land shall, in default of agreement, be referred to and determined by the War Damage Commission, whose decision shall be final; and paragraph 6 of the First Schedule to the War Damage Act, 1943 (which enables the Commission to regulate the procedure for the determination of questions subject to determination by them under that Act) shall have effect as if any question falling to be determined by the Commission under this paragraph were a question subject to determination by them under that Act.