The interior mirror in my Mark 11 Superb Estate apparently has an automatic dipping mirror

However whether I switch the light on or off doesn't appear to make any difference to the image I see in the mirror

 

Am I doing anything wrong?

It is only effects the lights in the rear view mirror from a car that is behind you onto the mirror as that would dazzle which is not is happening with your interior light.

It's easy to test...

 

On the front facing side of the mirror, there is a small cutout with a light sensor. Turn your headlights on and cover the sensor with your hand, it should darken the mirror if it's on and working