31.—(1) At a preparatory hearing the judge advocate may make an order or ruling on—
(a)any question as to the admissibility of evidence;
(b)any other question of law, practice or procedure relating to the case.
(2) An order or ruling made under this rule shall have effect until the conclusion of the court-martial trial unless it appears to the judge advocate on application made to him at any stage during the proceedings that in the interests of justice it should be varied or discharged.