Complex sentence: In 1975, the New Opera Company in London produced the work which was conducted by Charles Mackerras, while the opera was first seen in the US in 1981 in a concert version given by the St. Louis Symphony in St. Louis, conducted by Leonard Slatkin.
Split into simpler sentences: In 1975, the New Opera Company in London produced the work under the baton of Charles Mackerras. In the USA the opera was first seen in 1981, in a concert version given by the St. Louis Symphony in St. Louis, conducted by Leonard Slatkin.