In the novel The Clocks, Hanbury was a counter-espionage agent working in the same department as Colin Lamb. He had been working on understanding the Larkin network, how the spy Larkin managed to get his documents couriered overseas to foreign intelligence. Just before the events in the book, Hanbury had been killed by a hit and run car in London (it is not stated whether this is just an accident or a deliberate act by his enemies). Colin Lamb took over the case.
Hanbury had in his wallet a piece of hotel notepaper on which he had drawn a crescent moon, the digits "61" and the letter "M". Larkin operated as a spy in the naval station at Portlebury. Lamb started out from there and moved outwards in an expanding circle, checking anything related to a crescent. Following this procedure was what led him to Wilbraham Crescent at the beginning of the events in the novel.