This is the Melos Ensemble of London, to not be confused with the german Melos Quartet, or the italian Melos Art Ensemble.
They disbanded in 1973, but reformed in 1974 featuring some of the original performers.
The Melos Ensemble is a group of musicians who started in 1950 in London to play chamber music in mixed instrumentation of string instruments, wind instruments and others. The ensemble's reputation for excellence has encouraged composers to write music exploring these resources. Benjamin Britten composed the chamber music for his War Requiem for the Melos Ensemble and conducted the group in the first performance in Coventry.
They should not be confused with two other chamber groups of similar name, the Melos Quartet or the Melos Art Ensemble (an Italian group).