Can someone give me a detailed summary of The Mystery of Three Quarters by Sophie Hannah?

Hercule Poirot returns home after an agreeable luncheon to find an angry woman waiting to berate him outside his front door. Her name is Sylvia Rule, and she demands to know why Poirot has accused her of the murder of Barnabas Pandy, a man she has neither heard of nor ever met.. She is furious to be so accused, and deeply shocked. Poirot is equally shocked, because he too has never heard of any Barnabas Pandy, and he certainly did not send the letter in question. He cannot convince Sylvia Rule of his innocence, however, and she marches away in a rage.Shaken, Poirot goes inside, only to find that he has a visitor waiting for him a man called John McCrodden who also claims also to have received a letter from Poirot that morning, accusing him of the murder of Barnabas Pandy.