I wonder if Joyce recognised Jo’s name as the daughter of a hitherto-unseen close friend, tells Catherine that Jo is X’s daughter who has lost touch with her since she married that awful bloke, X always suspected abuse but couldn’t get Jo to open up blah blah, which sends Catherine back to the house in the hope of getting Jo to talk to her while Evil PE Teacher is at work, and either finds the body or at least something suspicious.
To whoever asked upthread if Catherine isn’t allowed to hit the little scrote she was arresting because he hit her first…she can use reasonable force but two punches to the head wouldn’t be classed as “reasonable”. The police are trained in restraint and she’d be expected to use her taser or some other approved method rather than tit-for-tat violence which could have killed him. She was taking out the frustrations of an utterly shitty few days on him rather than acting as a dispassionate upholder of the law. I absolutely don’t blame her, but when she first punched him I actually moaned “oh no Catherine, he’s not worth your career”.