Hi Garry and welcome
It sounds like you have checked the right things. You're right, the coloured clutch knob affectes when the clutch is on. It has 3 positions (labelled on the machine):
Off: always off
Off / auto (central position): Off when the machine is upright, on when the body is tilted relative to the head
On / auto (forward position): The knob moves to this position when the machine is tilted and the brushbar turns on.
So to check the brushbar, select Off/Auto (middle position) when the machine is upright. Tilt the machine and make sure the knob changes to the forward position. With the machine body still tilted, lie the machine on the side and then turn it on to check. Keep clear!
Hope it works....