The 18-year-old X Factor runner-up has been cast in the role that made Jane Horrocks famous, playing a shy, lonely girl called Laura who has an impressive voice and can impersonate singers like Edith Piaf and Shirley Bassey.
Jane, 45, wowed in the role on stage in 1992 and took the show to the big screen in the 1998 hit movie version Little Voice, co-starring Sir Michael Caine.
Diana will begin rehearsing next month, and the show - by playwright Jim Cartwright - is due to start later in the autumn.