Complex sentence: After Lennon began a relationship with Yoko Ono, then Apple began to founder as the group grew apart, Shotton parted company with Lennon and the Beatles, resuming management of the Hayling Island supermarket, which he continued to run until the late 1970s.
Split into simpler sentences: After Lennon began a relationship with Yoko Ono, Shotton parted company with Lennon and the Beatles, then Apple began to founder as the group grew apart. He then resumed management of the Hayling Island supermarket, which he continued running until the late 1970s.