Father Brown states that it is the Ninth Commandment that makes lying a sin. For Catholics, "bearing false witness" is the Eighth Commandment.
As the cast prepared to assemble when Donald is released , although it is Christmas Eve in the story, deciduous plants are clearly in leaf and an Elderberry Bush can be clearly seen in flower to the left - which indicates that filming was in Late Spring.
Between 13:00 and 14:00 minutes in, Mrs. Clough speaks with Father Brown and the Inspector about her son in a room with windows that look on her garden. However, the garden is not there. Instead, the only thing visible is Green Screen.