Mr Barrett of Wimpole Street is a parallel used by Miss Marple towards the end of the novel Sleeping Murder.
The Mr Barrett Miss Marple is referring to is a character in The Barretts of Wimpole Street, a play written by Rudolf Besier in 1930 which was subsequently made into a film in 1934. If, as is often thought, Christie set Sleeping Murder the 1930s, it would be natural for Miss Marple and Gwenda Reed to be familiar with this play/film. The work is a fictionalized account of the love story between the poet Robert Browning and another poet and his eventual wife Elizabeth Barrett. The Mr Barrett being referred to is Edward Moulton-Barrett, the father of Elizabeth. He was a tyrannical and over-controlling man who terrorised his nine daughters and forbade them to associate with men or to be married.