Complex sentence: He is probably remembered best as producer of The Clash's masterful London Calling album, from 1979, the band themselves have always held up Stevens' input as a major factor in the albums popularity and quality.
Split into simpler sentences: He is probably best known as the producer of The Clash's masterpiece London Calling album, from 1979. The band themselves have always held up Stevens' input as a major factor in the album's popularity and quality.