"The closing music, which was largely transcribed from a Bulgarian folksong", according to Gary Carpenter. The three trumpet players at the end of the film were not LSO but Colin Clague, Cliff Haines (an older session player who lived in studios most of his career) and Stephen Hetherington. (Gary, 9/23/17 Facebook) Labelled as Sunset on the soundtrack album, it is a Bulgarian folk tune entitled Zamrakanala Mona Jana. The principle trumpet holding out that long note was Colin. (Gary, 9/23/17, Facebook)
The Silva Screen version lists it as being played by The City of Prague Philharmonic Orchestra, Gary Carpenter, arranger.
It is the same in all movie versions and audio recordings.