The Missa Luba was certainly the main musical element in the film. I cannot recall whether that was what was in the background as the boys left the chapel or not, however.
Thanks for the input people but... its the piece that is played on the organ as they leave the chapel. DA DA DA, DA DADADA, DA DADADA, DA DADADA, DAAA DAAA. Its very uplifting and I think the composer starts with a C. I remember it at college. It was always played after we left the chapel on a sunday ( any worksop college people out there? )
I'm not doubting anyones answers, but wikipedia claims...
"A single piece of music recurs in the film, the "Sanctus" from the Missa Luba. This version of the Latin Mass in African style, sung by a choir of Congolese children, had been on the UK Singles Chart in the 1960s."