4.—(1) Subject to the provisions of this regulation, any participant in the principal scheme may become a contributor to the AVC scheme by making written application in such form as the Trustees shall require and by having such application accepted.
(2) A contributor may not make any contributions to the AVC scheme after he has ceased to be a participant in the principal scheme, but may make a further application under paragraph (1) above if he again becomes a participant.
(3) The Trustees may with effect from such date as they may determine close the AVC scheme to participants in the prinicipal scheme who are not contributors to the AVC scheme at that date.