No third series
Despite the optimism of a Trivia entry ("The series is planned to be a 5-season story arc, taking place between the years 1932 and 1947"), it appears that there will not be a third series, though the only confirmation I can find from an admittedly brief Google is on Facebook.
I found it dull, with hardly any likeable characters (in fact I can't think of one),the only redeeming feature being the scenery. "Constipated" is one word that others have used to describe it. I've stuck with it,but won't be sorry to see it end.
Not that characters have to be likeable to make a good drama - there are many TV and film "baddies" who appeal - but only Julie Walters and Patrick Malahide aroused any interest in me - and I'm a bit curious about the Sikh policeman and his loyalties.