Factual error: A bike rider is warming down on freestanding rollers and is apparently thrown off when an assailant sticks a wrench in the front wheel. That is not how physics work. If a bike is in motion, the forward momentum of the bike and rider will absolutely throw the rider head over heels. But on freestanding rollers, there is no momentum. The bike and rider are standing still and only the wheels are moving. The rider will, of course, topple off but not be thrown.
Continuity mistake: When Helen Grace is in the retired GP's house choosing which apple she will eventually stuff in to her victim's mouth, her hand wavers over one apple, but instead chooses another. At the end when we see a flashback of that scene, she picks up the first apple she was originally going for.
Plot hole: Several shots show Barnaby reviewing a newspaper announcing a new death in Strangler's Wood, the Brazilian Woman. No one supposedly knows who she is, as the article states, but the photo of the dead woman is a professional headshot from when she was alive. How could they have obtained her headshot if no one knows who she is?