21.—(1) The Tribunal may direct that any preliminary question of fact or law which appears to be in issue in relation to the appeal, be determined at a preliminary hearing.
(2) If, in the opinion of the Tribunal, the determination of that preliminary question substantially disposes of the appeal, the Tribunal may—
(a)treat the preliminary hearing as the hearing of the appeal; and
(b)make such order by way of disposing of the appeal, as it thinks fit.
(3) If the parties so agree in writing, the Tribunal may determine the preliminary question without an oral hearing.
(4) Where the Tribunal determines the preliminary question without an oral hearing under paragraph (3), the Tribunal must not at the same time dispose of the appeal, unless the parties have agreed in writing that it may do so.