20.—(1) This Article applies where a court passes a custodial sentence other than one fixed by lawF1. . . [F2 or falling to be imposed under[F3 Article 70(2)] of the Firearms (Northern Ireland) Order[F3 2004]].
(2) The custodial sentence shall be—
(a)for such term (not exceeding the permitted maximum) as in the opinion of the court is commensurate with the seriousness of the offence, or the combination of the offence and one or more offences associated with it; or
(b)where the offence is a violent or sexual offence, for such longer term (not exceeding that maximum) as in the opinion of the court is necessary to protect the public from serious harm from the offender.
(3) Where the court passes a custodial sentence for a term longer than is commensurate with the seriousness of the offence, or the combination of the offence and one or more offences associated with it, the court shall—
(a)state in open court that it is of the opinion that paragraph (2)(b) applies and why it is of that opinion; and
(b)explain to the offender in open court and in ordinary language why the sentence is for such a term.
(4) A custodial sentence for an indeterminate period shall be regarded for the purposes of paragraphs (2) and (3) as a custodial sentence for a term longer than any actual term.