The phone that Jones takes the call from Mrs. Fox on has a curly chord. These weren't invented until much later than the series is set.
In the first episode, Sargent Wilson says that he was a Sargent in the Royal Artillery in the First World War. However, in this episode he says that he was a Captain.
When on the phone to Mrs. Fox, Mainwaring calls her Mrs. Wilson.
Mrs fox is now called "Mildred" where in Mums Army she was called "Marcia"