The cassette recording does not match the words spoken onscreen by Judith's killer.
Barnaby tells someone that a recorded confession cannot be used because it constitutes entrapment, but this is not what entrapment means. Entrapment occurs when a law official encourages someone to break the law, not when they secretly record a confession. It is true that secret recordings by legal officials are also frowned upon in some (but by no means all) jurisdictions, but in this case the recorder was not a legal official, so the recording would most likely have been admissible.
Not possible for the full moon to go to half moon in one day.
At the beginning of the episode, the two leading cyclists enter the town at the end of the race. They are being filmed because the programme is being shown on the pub's TV. One of the angles on the TV shows the lead cyclist from the side pedaling, so obviously a TV camera must be travelling alongside the riders at the same speed. However, other camera angles clearly demonstrate that there is no such TV crew doing this.