Hi Telbg,
As far as i an aware HDC will only be active in first or reverse gears. When active, the HDC light on the dashboard will come on. IF you are not in the correct gear, the indicator light will flash.
To use HDC :
Stop the car at the top of the hill that you wish to descend, select first gear and switch on HDC. Get the car moving slowly. As you go down the hill, the car will try and maintane a speed of no more than 5 mph and will activate the brake on the car when it needs to.
The ca'rs brakes will work as normal when the pedal is pushed. If the clutch is disengaged for 30 seconds the HDC light will flash, 60 seconds cause the fault light to come on.
You can leave HDC turned on whilst driving around off road.
Hope this helps
Dave