Thanks for asking:
Following the 1987 market crash, there will be a struggle for power within the Labour party.
You need to choose to play as the reformist faction and then win the struggle for power.
As the reformists, you win the struggle for power by bringing the traditionalist's influence to be less than 50% and then triggering a vote of no confidence by bringing party support below 20%.
If you do this, you will get a special event where Lange is forced to resign and Roger Dogulas and his faction assume control of the party.