If 25th December is the first day of Christmas then the fifth day of Christmas must be 29th December and not 30th December.
Inspector Ames (Ewan Roberts) describes the late Johhny Blake as being 160 pounds in weight, as in the American system. The 1960's were before the current metric system came into being and weights would have been 'stones, pounds and ounces'. Ames, therefore, should have said 14 stone, 6 pounds.
Inspector Ames (Ewan Roberts) talks of Scotland Yard's "Homicide Department". Scotland Yard does not have a "Homicide Department"