The Three Taps :

Holidaying in a country inn, wealthy Jephthah Mottram dies, poisoned by gas. But Mr Mottram was insured with the massive Indescribable Insurance Company, under its famously extravagant Euthanasia policy: with half a million pounds payable on death... except in the case of death by suicide. The Indescribable's somewhat desultory private detective, Miles Bredon, along with his wife Angela, is sent to investigate. Was Mottram's death suicide, or something more sinister?