Incidental music means background score, but in this instance it is well documented that Bryan Forbes called on Barry to supply 2 pieces of jazz for a nightclub sequence featuring a trumpet. The tunes are very catchy and I'm surprised they've never been released commercially. Forbes was impressed and employed Barry to score his next five films. Would have been more but Barry couldn't score Madwoman of Chaillot and Raging Moon's producer wanted Stanley Myers, both wonderful scores.
I seem to recall that the music was mostly Rachmaninoff (one of the popular concertos). That struck me as an odd choice for a movie that was otherwise aspiring to the "kitchen sink" working-class realism of its period. But this is a very old memory and perhaps fallible.