It is very well known that Noel Coward himself chose Rachmaninov's 2nd Piano Concerto to be used as underscoring for this film, in spite of Muir Matheson's objections. Eileen Joyce plays on the soundtrack in performances recorded especially for the film and conducted by Matheson.
However, I have never been able to find out who it was who made the music cutting decisions as to which parts of the concerto were to be used - and so skilfully matched - to the action on screen.
Matheson was a conductor and adept at conducting music to film (which is why he is credited on nearly every British film of this period). But I do not believe he was so skilled as to make the editorial decisions for the concerto's use here, which are often as skilled and imaginative as say, Korngold's use of Mendelssohn in A MIDSUMMER NIGHT's DREAM.
IMDB lists John Hollingsworth as the associate musical director (uncredited) who was another conductor, later to work memorably at Hammer, and who died of pneumonia aged only 47 in 1963. Was it him? I believe whoever it was, had to have experience as a composer or arranger. I am not sure that either Matheson or Hollingsworth did.
What do the film music experts here say?
Open, general discussion of music during the era of classic/nitrate movies