4.—(1) In any case where a prospective apportionment is to be made by arbitration, an arbitrator shall be appointed—
(a)where regulation 11 applies, by the President;
(b)where regulation 10 applies, by agreement between the occupier of the holding to which the prospective apportionment relates and any other interested party, or, in default, by the President on an application by that occupier.
(2) Where sub-paragraph (1)(b) applies, the occupier shall give notice to the Department of the appointment of the arbitrator pursuant to the agreement, or of the application to the President for the appointment of an arbitrator.