In the novel The Body in the Library, Muswell is the chauffeur of Colonel and Mrs Bantry. Dolly sent him to drive Miss Marple to Gossington Hall, so she could see the body in the library.
Muswell is not featured in either the BBC 1984 adaptation of the novel or the ITV 2004 adaptation. In the 1984 adaptation, there is a scene where Colonel Bantry drives out to Venn's Quarry but the butler Lorrimer is the driver. In the 2004 adaptation, Miss Marple walked to Gossington Hall.