Time for Murder — Kenneth Benton

Lord Harcourt, a retired High Court judge, has been found murdered in his bath - with the door locked on the inside. Chief Inspector Harriman discovers the startlingly ingenious method used to kill him, but at first cannot find either a credible motive or even a suspect. Then he realises that those in the old country house at the time all hold - separately and unknowingly - the keys to the mystery. But for how long will they survive?