Originally Posted by ATNotts
As I'm sure you know it is not an offence to listen to people doing their job
Actually it is.
Wireless Telegraphy Act, Part 2, Chapter 5, Section 48:
Interception and disclosure of messages
(1) A person commits an offence if, otherwise than under the authority of a designated person—
(a) he uses wireless telegraphy apparatus with intent to obtain information as to the contents, sender or addressee of a message (whether sent by means of wireless telegraphy or not) of which neither he nor a person on whose behalf he is acting is an intended recipient, or
(b) he discloses information as to the contents, sender or addressee of such a message.