Hoover Slalom SL8123 roller not turning

I have a Hoover Slalom SL8123 that every time the hoover is used in the near upright position the belt driving the roller goes slack. When it is in the horizontal position it works fine and the belt is tight. I have tried taking it apart but even removing the 7 screws underneath and removing the roller I can’t work out how to separate the top and bottom housing to see what is going on.