police get there, a man has been stabbed, there is a weapon and a motive.
the man who is accused of the stabbing is also at the scene.
why on earth would the police NOT arrest him?
Stabbing isn't attempted murder. The police had no right to suspect him of attempted murder until they'd established that he wasn't acting in self defence. Given that his son was there already beaten, the damage to his van and the fact there were five known troublemakers there, including the victim, there was no reason to suspect attempted murder. Mr Philpott's arrest was just an easy one to take the police one closer to their target for the month.