Complex sentence: At the beginning of 1967, Barrett was dating Jenny Spires (who would later marry future Stars member Jack Monck), however, unknown to Barrett, Spires had an affair with Peter Whitehead.
Split into simpler sentences: At the beginning of 1967, Barrett was dating Jenny Spires (who would later marry future Stars member Jack Monck). However, unknown to Barrett, Spires had an affair with Peter Whitehead.