we have a disease on our box hedging

They're either Wooly Aphids or Mealybugs, probably Aphids... If you don't oppose to using chemicals Provado Ultimate Bug Killer is a good choice.

You could also try blasting them off with a jet of water.

It's probably powdery mildew, spray with a fungicide from the garden centre.

I agree with Bamboo. If it is white and powdery it will be a fungal bloom, likely the mildew type. These things tend to attack when a plant is stressed in some way and thus weakened. Is your hedge droughted? If there is a chance it is dry at the roots then give a good deep watering with a hose quickly, and like Bamboo says, use an appropriate fungucide also.