Complex sentence: Heymer became his lover as well as manager, and stayed with Howerd for more than thirty years, until Howerd's death, with Heymer helping to revive Howerd's flagging career in the 1960s.
Split into simpler sentences: Heymer became his lover as well as manager, and stayed with Howerd for more than thirty years, until Howerd's death. With Heymer helping to revive Howerd's flagging career in the 1960s.