6. (1) In proceedings for an offence under this Act it shall be a defence for the defendant to prove—

(a) that he or she—

(i) was required by law to obey the orders of a government or of a superior, whether military or otherwise, and

(ii) was acting under an order of that government or superior,

and

(b) that he or she—

(i) did not know that the order concerned was unlawful, or

(ii) believed it to be lawful.

(2) Subsection (1) does not apply to a defendant where the order concerned was manifestly unlawful.