Complex sentence: A childhood friend of the Kray twins, Cornell was a prominent criminal in east London during the 1960s and upon moving to South London joined up with the Kray twins' rivals, the Richardson Gang, who were brothers Charlie Richardson and Eddie Richardson.
Split into simpler sentences: A childhood friend of the Kray twins, Cornell was a prominent criminal in east London during the 1960s. Upon moving to South London joined up with the Kray twins' rivals, the Richardson gang, led by brothers Charlie Richardson and Eddie Richardson.