(1) In proceedings for an offence under section 47(1)(c) or (d) relating to an object it is a defence for the accused to show that he did not know and had no reason to believe that the object was a nuclear weapon.
(2) But he shall be taken to have shown that fact if-
(a) sufficient evidence is adduced to raise an issue with respect to it; and
(b) the contrary is not proved by the prosecution beyond reasonable doubt.
(3) In proceedings for such an offence it is also a defence for the accused to show that he knew or believed that the object was a nuclear weapon but, as soon as reasonably practicable after he first knew or believed that fact, he took all reasonable steps to inform the Secretary of State or a constable of his knowledge or belief.