Complex sentence: Sassoon was the only child of the poet Siegfried Sassoon and Hester Sassoon (née Gatty), when their marriage was already in difficulties, and in 1947 they separated.
Split into simpler sentences: Sassoon was the only child of the poet Siegfried Sassoon and Hester Sassoon née Gatty. He was born at a time when his parents' marriage was already in difficulties, and in 1947 they separated.