Complex sentence: Sullivan was serving as the Festival's musical director for the third time, and had already produced a large work for chorus and orchestra for his first Leeds Festival, "The Martyr of Antioch" (1880).
Split into simpler sentences: Sullivan was serving as the Leeds Festival's musical director for the third time. Sullivan had already produced a large work for chorus and orchestra for his first Leeds Festival, "The Martyr of Antioch" (1880).