I plan on changing the belt and water pump on my M16. Here's the rover tech video

Rover - Rover Professionals - Technicians Update 3 (1993) https://youtu.be/2UhWenKlhGE

Interesting note at the end. total new belt deflection should be less than 6mm. The gauge is only used as a reference which was a change. Anyway, the 6mm deflection between the cams from horizontal was interesting (haynes claims 19mm until you can get to your rover dealer!!).