In the novel Death in the Clouds, 315 Grosvenor Square is the London townhouse of the Earl and Countess of Horbury. Lord Horbury spends most of his time at Horbury Chase and his wife mostly lives in the townhouse.
In the course of the story, Poirot sent Norman Gale to Grosvenor Square disguised as one "Mr Robinson". He role-played a blackmailer who has documents which belonged to Madame Giselle which would expose Lady Horbury's adultery with Raymond Barraclough. In exchange for the documents, "Robinson" demanded a large sum which Lady Horbury could not possibly pay. Shortly after this, Poirot himself arrives at Grosvenor Square on the pretext that he was there on behalf of "Mr Barraclough". Because "Mr Robinson" had already softened Lady Horbury up, she was receptive to Poirot's offer to help rid her of the blackmailer. She therefore unloaded to Poirot the entire background of her relationship with Madame Giselle.
Grosvenor Square is an actual location in London's fashionable Mayfair district which also occurs in other of Christie's novels, but it is not clear if the house numbers run up to 315.