When the gardening expert is shown filming his TV show, the crew in the background are actually the real Midsomer Murders production crew, including the director hunching over his monitor.
During the scene in the tent at the festivities, the community youth orchestra is doing their best to actually play the Midsomer Murders theme.
While visiting the home of one of the suspects, DCI Barnaby points out that the place is called Lothlorian, and explains to DS Troy that it is from The Lord of the Rings (where it is spelled Lothlórien, by the way). Orlando Bloom, who plays Peter Drinkwater, later starred as Legolas in The Lord of the Rings trilogy directed by Peter Jackson.
Marcus Devere twice says he was going the have a chat with Peter Drinkwater "mano a mano" meaning a talk "man to man". This is an incorrect use of the phrase based on the fact that mano sounds like and is spelled similarly to man. "Mano a mano" actually means "hand to hand" and is supposed to indicate a physical altercation. While you could say "we're going to have to settle this dispute mano a mano", this would suggest settling the matter via fisticuffs. Another correct usage might be, "Despite technology making it possible to wage modern warfare at ever-increasing distances, the U.S. Army still trains its combat soldiers in hand-to-hand combat because while fighting in urban areas, a soldier may have to fight an enemy mano a mano"