Being a Theatre buff too. I was idling through Theatre World for 1962, and spotted some familiar names. John Addison wrote the music for the London West End productions of SEMI-DETACHED. BLOOD OF THE BAMBERGS and UNDER PLAIN COVER. Richard Rodney Bennett. CURTMANTLE. James Bernard. VIRTUE IN DANGER. Clifton Parker. THE SILVER CURLEW and LOVES LABOUR LOST. Brian Easdale. MACBETH. Richard Addinsell JOYCE GRENFELL REQUESTS THE PLEASURE..(now who else can I find)and anyone know of other composers who composed for Theatre?
Going back a ways further, Alex North composed the scores for a number of plays; most notably, his scores for the theater productions of "Death of a Salesman" (1949) and "The Innocents" (1950) brought him an invitation to Hollywood to score the film version of "A Streetcar Named Desire."