(1) An authorisation under section 48 has effect, subject to subsections (2) and (3), during the period specified in the authorisation.
(2) The period specified shall not exceed 28 days.
(3) An authorisation may be renewed in writing by the person who gave it or by a person who could have given it; and subsections (1) and (2) shall apply as if a new authorisation were given on each occasion on which the authorisation is renewed.