Why Didn't They Ask Evans?

While out playing golf with his friend Dr. Thomas, Bobby Jones finds a man who has apparently fallen over the side of a cliff onto the seaside rocks below. His dying words are "Why didn't they ask Evans?". The dead man is identified by his sister Mrs. Cayman as Alex Pritchard, who had recently returned to the UK from abroad and was on a walking tour. Bobby and his friend Lady Frances Derwent soon conclude that illicit drugs are at the core of the mystery and suspicion falls on a certain Dr. Nicholson who runs a country clinic for drug addicts. The mystery revolves around a will and numerous false or stolen identities. But just who is Evans and what should he/she have been asked?