Sch 5 para 2
An appointment must be in writing and— (a) may relate to any particular appeal or matter specified in the appointment or to appeals or matters of a specified description; (b) may provide for any function to which it relates to be exercisable by the appointed person either unconditionally or subject to the fulfilment of any conditions specified in the appointment; and (c) may, by notice given to the appointed person, be revoked at any time by the Secretary of State in respect of any appeal or matter which has not been determined by the appointed person before that time.