Although a likable series and this episode has an interesting plot, two things in this flash back episode clash with the current story line:
1) In the first series Eliza and William act as if they have never met before. Complete strangers to each other.
2) the original Duke as in the title makes a reappearance after being out of the series since season 2; the original Duke was the financial benefactor for Miss Scarlet in the first series.
It was really odd when the inspector was addressed three times in a short time near Scotland Yard as "the Duke" in an episode; as if every one knew his nick name and always addresses him with that nick name.
Wish they did not make those stupid changes, make you cringe, really annoying,