20.—(1) Where a plate is placed in combination with a sign shown in a diagram in Schedules 1 to 5 or 12, and that sign is illuminated in accordance with regulation 18, the plate shall, subject to paragraph (2), be illuminated by the same means as the sign.
(2) Paragraph (1) shall not apply where the means of lighting provided for the illumination of the sign adequately illuminates the plate.