In 1949, he appeared in "The Heiress" at the Theatre Royal, Haymarket opposite Ralph Richardson and Peggy Ashcroft, directed by John Gielgud. In his Sky Arts documentary series "Great West End Theatres", Sinden said that the play ran for 644 performances (19 months) and he was the only member of the cast not to have missed a performance: "As the play is the longest run in the theatre's history, I therefore gave more consecutive performances in this theatre than any other actor since it was built in 1820." The management gave him an engraved silver ashtray as a present in recognition of the fact, which he showed in the episode.