The Footsteps at the Lock :

Two cousins, Derek and Nigel Burtell, famously detest one another. Derek Burtell stands to inherit £50,000 under the will of his grandfather at age 25 - unless his drug-taking and dissolute lifestyle kill him first, in which case Nigel inherits instead. Dismayed by the latter possibility, Derek's many creditors insure his life with the vast Indescribable Insurance company, and Derek is advised to take a river trip for his health; and curiously it is Nigel whom he chooses to accompany him. So when Derek goes missing on the river, the Indescribable is quick to send its resident sleuth, Miles Bredon, to look into the circumstances...