At numerous times throughout series 1 notably in "One Last Dance" and "Traitor's Gate" close-ups of intelligence files show that all the information is the same and all relates to Mary Kane's terminated pregnancy from episode 1 "Thou Shalt Not Kill".
In many territories including the United States, this series is titled "MI-5". The convention for writing the name of this organisation is actually "MI5" - no hyphen or space. The same goes for the sister organisation that employs James Bond: MI6.