This is quite possibly the only example in television history of an episode that is simulatenously a series finale, a "Christmas" special episode, the middle of the overall combined story arc, and the backdoor pilot for the continuation of the evolution of the following series' dynamic as the sequel series.
The rise of Jim Hacker to party leader and, therefore, Prime Minister to avoid a split in his party has so many similarities to the rise of John Major a few years later, the writers must have had a crystal ball.
The date on the Evening Standard newspaper shown as Hacker is on his way to make his speech, is clearly dated as Tuesday January 15th, 1985 which is the correct dating for that year.