16Extra safeguards in the case of certificated warrants
(1)For the purposes of section 15 the requirements of this section, in the case of a warrant in relation to which there is a section 8(4) certificate, are that the intercepted material is read, looked at or listened to by the persons to whom it becomes available by virtue of the warrant to the extent only that it—
(a)has been certified as material the examination of which is necessary as mentioned in section 5(3)(a), (b) or (c); and
(b)falls within subsection (2).
(2)Subject to subsections (3) and (4), intercepted material falls within this subsection so far only as it is selected to be read, looked at or listened to otherwise than according to a factor which—
(a)is referable to an individual who is known to be for the time being in the British Islands; and
(b)has as its purpose, or one of its purposes, the identification of material contained in communications sent by him, or intended for him.
(3)Intercepted material falls within subsection (2), notwithstanding that it is selected by reference to any such factor as is mentioned in paragraph (a) and (b) of that subsection, if—
(a)it is certified by the Secretary of State for the purposes of section 8(4) that the examination of material selected according to factors referable to the individual in question is necessary as mentioned in subsection 5(3)(a), (b) or (c); and
(b)the material relates only to communications sent during a period of not more than three months specified in the certificate.
(4)Intercepted material also falls within subsection (2), notwithstanding that it is selected by reference to any such factor as is mentioned in paragraph (a) and (b) of that subsection, if—
(a)the person to whom the warrant is addressed believes, on reasonable grounds, that the circumstances are such that the material would fall within that subsection; or
(b)the conditions set out in subsection (5) below are satisfied in relation to the selection of the material.
(5)Those conditions are satisfied in relation to the selection of intercepted material if—
(a)it has appeared to the person to whom the warrant is addressed that there has been such a relevant change of circumstances as, but for subsection (4)(b), would prevent the intercepted material from falling within subsection (2);
(b)since it first so appeared, a written authorisation to read, look at or listen to the material has been given by a senior official; and
(c)the selection is made before the end of the first working day after the day on which it first so appeared to that person.
(6)References in this section to its appearing that there has been a relevant change of circumstances are references to its appearing either—
(a)that the individual in question has entered the British Islands; or
(b)that a belief by the person to whom the warrant is addressed in the individual’s presence outside the British Islands was in fact mistaken.