"Information held by a public authority is exempt information if it has at any time been held by the authority for the purposes of- (a) Any investigation which the public authority has a duty to conduct with a view to it being ascertained- (i) whether a person should be charged with an offence, or (ii) whether a person charged with an offence is guilty of it. (b) Any investigation which is conducted by the authority and in the circumstances may lead to a decision by the authority to institute criminal proceedings which the authority has power to conduct, or (c) Any criminal proceedings which the authority has power to conduct."