The Fourth Dimension
World War Three
The date given on the UNIT website along with the passage of time onscreen, suggests that the story is set one year in the future from the date of broadcast.
Until the very last minute this episode was to be named Aliens of London Part Two. All episodes of subsequent multi-part stories have been given individual titles.
The destruction and rebuilding of 10 Downing Street in this episode is referenced in The Sound of Drums, where the Master sets up in the newly-built office.
Harriet Jones reappears in The Christmas Invasion, by which time she has been elected Prime Minister. Despite falling out with the Doctor, Harriet returns in The Stolen Earth, surrending herself to the Daleks in order to save mankind.
Did you notice the word 'Salford' graffitied on the wall outside Mickey's flat? and did you also know that this is the hometown of actor Christopher Eccleston?