Sgt. Vaill says that the first man he killed in battle was Algerian. During World War One, Algeria was a colony and ally of Sgt. Vaill's country, France.
This episode is set in 1915. The captain's service revolver is an Enfield No 2 Mk 1*, which would not be produced until 1938.
In the introduction, John Newland refers to a British mortar unit in Flanders and then points to an area in eastern France near the German and Swiss Borders. The only region of France known as Flanders is in the very north along the border with Belgium.