At the end of "The Six Thatchers" (Season 4 episode 1) Sherlock is on the bridge and deduced that
The “English woman” is Mrs. Norberry, the secretary.
How did he come to this conclusion? I see absolutely no real clue or evidence that leads to her!
During the scene, Sherlock sees a few scenes in his head:
- The A.G.R.A. burglar saying: “You think you understand? You understand nothing.”
- Images of the Thatchers breaking
- His brother saying "code names Antarctica, Langdale, Porlock and Love"
- Mary talking about how receptionists know everything, hear everything
It seems to me that it's just a guess like "Hey, Mary told me about a receptionist earlier, this woman has a secretary, it must be her". That would be disappointing for Sherlock, so I hope there is another reason.