I think section 2 of the OSA 1989 covers it (and to save anyone else the bother, our signatures were obtained as a reminder but the acts apply with or without one)
Defence.
(1)A person who is or has been a Crown servant or government contractor is guilty of an offence if without lawful authority he makes a damaging disclosure of any information, document or other article relating to defence which is or has been in his possession by virtue of his position as such.
(2)For the purposes of subsection (1) above a disclosure is damaging if—
(a)it damages the capability of, or of any part of, the armed forces of the Crown to carry out their tasks or leads to loss of life or injury to members of those forces or serious damage to the equipment or installations of those forces; or
(b)otherwise than as mentioned in paragraph (a) above, it endangers the interests of the United Kingdom abroad, seriously obstructs the promotion or protection by the United Kingdom of those interests or endangers the safety of British citizens abroad; or
(c)it is of information or of a document or article which is such that its unauthorised disclosure would be likely to have any of those effects.