After waiting a week to see what happened I had a look at my Oak leaves which were sprayed as an attempt to treat the heavy powdery mildew. I am so extremely surprised at how effective this has been. The underlying damage to the leaves is obviously still there - but the mildew is almost entirely gone.
This should extend my growing season a little and improve the vigour of the oaks greatly.
This is a great treatment. I cant see any downsides.