Complex sentence: The Reading Festival, the original and senior of the two, is the world's oldest popular music festival still in existence having been established in 1961 by Harold Pendleton, who had set up the Marquee Club in Soho 4 years earlier.
Split into simpler sentences: The Reading Festival, the original and senior of the two, is the world's oldest popular music festival still in existence. It was established in 1961 by Harold Pendleton, after he had set up the Marquee Club in Soho 4 years earlier.