The Footsteps at the Lock

At first sight the case looks simple enough to private investigator Miles Bredon. Two cousins on a boat trip on the River Thames: Derek with a £50, 000 reason for surviving the next two months until he inherits a legacy; Nigel with a £50, 000 reason for getting rid of him and inheriting the money himself. When Derek disappears, Nigel naturally falls under suspicion - not least because he has a train of alibis that is almost too perfect.
But where is the body? And if this is not murder, whose is the photograph of a body slumped in a boat, and who left the wet footprints at the lock?