This would be better if...
1) It stopped the heavy-handed preaching. Yes, women were unfairly treated in the professional world of the 1950s (and are still under represented today), but a contemporary tv series can't retrospectively change social attitudes. And why set something in the past only to point out how awful it was?
2)It managed to have some mock-historical footage, like "Zelig", or "Forrest Gump". I'm not sure how much this would cost, or how difficult it is to make it look convincing, but it would certainly add to the verisimilitude of the series if, for example, we "saw" Dominic West interviewing Eden or MacMillan.
3) More "real" historical figures - may be jumping the gun a bit, but it'd be a nice touch if, in investigating the conspiracy, the journalists spoke to Kim Philby, or, given his journalistic connections, Guy Burgess.
4) The characters were a bit more likeable. Wishaw's Freddie Lyon comes across as a disingenuous creep, West's Madden a bit of a sleeze-ball, and Garai's Rowley a bit holier-than-thou.