Complex sentence: Kerr is also frequently referred to in The Kenneth Williams Diaries edited by Russell Davies and it seems he was very fond of her and they were good friends who also worked with each other in theatre productions.
Split into simpler sentences: Kerr is also frequently referred to in "The Kenneth Williams Diaries" edited by Russell Davies. It seems he was fond of her and they were friends, and they also worked with each other in a number of theatre productions.