Complex sentence: He was one of the most successful manager-producers in the West End during the middle of the 20th century, indeed the director Tyrone Guthrie commented that, in his prime, Beaumont had the power to make or break just about any theatrical career in London.
Split into simpler sentences: He was one of the most successful manager-producers in the West End during the middle of the 20th century. The director Tyrone Guthrie commented that, in his prime, Beaumont had the power to make or break just about any theatrical career in London.