Complex sentence: In his last year at the Royal Academy of Music the Quartet was founded, and it continued during the time he was in the London Symphony Orchestra from 1976-1980, after which he left the Orchestra in order to dedicate himself full time to the quartet.
Split into simpler sentences: In his last at the Royal Academy of Music the Quartet was founded, and it continued during the time he was in the London Symphony Orchestra from 1976. He left the Orchestra in 1980 to dedicate himself full time to the quartet.