Dr Haydock is a physician living in St. Mary Mead.
Before moving to St. Mary Mead he had a practice near Westmoreland. The he got to know Lucius Protheroe.
In The Body in the Library, it is mentioned that Dr Haydock is also the police surgeon. He arrives at Gossington Hall to examine the body found in the library. He puts the time of death as between ten 'o clock and midnight. He also tells Colonel Melchett that the girl was drugged with a powerful narcotic before being strangled.
In The Mirror Crack'd from Side to Side, Dr Haydock has semiretired, but still attends some of his old patients. He has a partner, Dr Sandford. When Miss Marple tells him that she is having difficulty knitting, as she drops stitches all the time, and very often does not know that she has dropped them, he recommends that she try unravelling instead, and say that he prescribes "a nice juicy murder".
Later in the novel, Miss Marple calls Dr Haydock to tell him that she is perfectly well, and does not need anyone to stay in the house with her, as long as someone comes in daily to do the cleaning. However, Dr Haydock says that to stay alone in a house at her age is a risk. He sees that Miss Knight irritates her, and advises her to find an old servant of hers, someone she likes and who has lived with her before. As he is leaving, he mentions some cases of German measles, whooping cough, and a suspected case of scarlet fever that he has to attend to, in addition to his regular patients. This gives Miss Marple an idea about the motive for the murder.
- The Murder at the Vicarage (1930)
- The Body in the Library (1942)
- The Case of the Perfect Maid (1942)
- The Mirror Crack'd from Side to Side (1962)
- Sleeping Murder (1976)
- The Case of the Caretaker (short story)
- Death by Drowning (short story)
- In the 1986 Miss Marple adaptation The Murder at the Vicarage, Dr Haydock is played by actor Michael Browning.
- In the 2004 Agatha Christie's Marple episode The Murder at the Vicarage, Dr Haydock is played by actor Robin Soans.
- In the 2004 Agatha Christie's Marple episode The Murder at the Vicarage, Dr Haydock is played by actor Robert Powell.
- In the 2010 Agatha Christie's Marple episode The Mirror Crack'd from Side to Side, Dr Haydock is played by actor Neil Stuke.