(1) As soon as a fixed-term prisoner, other than a prisoner to whom section 243A , 244ZA, 244A , 246A , 247 or 247A applies, has served the requisite custodial period for the purposes of this section, it is the duty of the Secretary of State to release him on licence under this section.
(1ZA) Subsection (1) does not apply if—
(a) the prisoner’s case has been referred to the Board under section 244ZB, or
(b) a notice given to the prisoner under subsection (4) of that section is in force.
(1A) Subsection (1) does not apply if the prisoner has been released on licence under section 246 or 248 and recalled under section 254 (provision for the release of such persons being made by sections 255B to 255C).
(2) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
(3) For the purposes of this section “the requisite custodial period” means—
(a) in relation to a prisoner serving one sentence, one-half of his sentence,
(b) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
(c) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
(d) in relation to a person serving two or more concurrent or consecutive sentences ... , the period determined under sections 263(2) and 264(2B) or (2E).
(4) This section is subject to paragraphs 5, 6, 8, 25 and 28 of Schedule 20B (transitional cases).