That Yew Tree's Shade by Cyril Hare

Francis Pettigrew, a former barrister and sometimes amateur detective, is plucked out of what promises to be a peaceful retirement in the Home Counties to deputise for the County Court judge. The proceedings offer him some unexpected insights into the lives of the new neighbours that he has - until now - only observed through his field glasses.

When the body of a penniless widow known for her good works is found on Yew Hill, a famous local beauty spot, Pettigrew is drawn into the case as a witness. Despite his best efforts to leave the inquiry to the police, it is he, with the unconscious help of a teenage boy, who puts the finishing touches on the solution.