Hi Cumbrianlass and welcome to GoY, good to know you want to avoid using chemicals. You could try the method given on this website which uses baking soda

http://gardening.about.com/od/gardenproblems/qt/PowderyMildew.htm

This is another good website with info on organic ways of treating not just mildew but other fungal infections

http://www.ghorganics.com/page15.html#Apple%20Cider%20Vinegar%20Fungicide

Heard good things about 9 parts water to 1 part full cream milk as a spray.
Personally I use Copper sulphate as a fungal spray, it is reasonably organic.

I tried the milk/water one last year to no effect at all. Maybe it was because I was using semi-skimmed?