This is a bit more like the England I remember from the old days, the days of Jack the Ripper and other eccentric murderers, rather than slums full of terrorists and muggers, and police arresting citizens for carrying penknives or talking on cell phones while driving.
Agatha Christie could have written this story, with its neighboring wives hanging over the backyard fence, discussing poison dosages. How veddy, veddy British!
I wonder if at trial the neighbor will be asked "and when you became aware that Mrs. Knight was planning to murder her husband, did you consider going to the police? If not, why not?" And if so, why didn't you?"
Nice neighbor.