hi susie...

I haven't got any answers , but this is more to ask is it just like a white pattern effect on the leaves ? We also have this ..but it hasn't stopped the growth of the courgettes . So I will be interested to read now what it is , should we remove it and how to treat it.

Some varieties of courgettes naturally have a silvery pattern on the leaves, or occasionally the whole leaf turns silvery. If this is the case, the silver is subtly metallic, like brushed aluminum, and usually forms a distinct pattern around the veins of the leaf.
Powdery mildew looks like random smudges of white flour on the leaves, and badly affected ones can turn yellow and brown under the powder. The easiest cure that I have found is to spray the leaves weekly with a mixture of 2 teaspoons of baking soda, and 1 tablespoon of organic washing up fluid, in a gallon of water. It often helps to let the mixture to sit overnight so the baking soda is thoroughly dissolved. The powder won't go away, but new growth will be unaffected. There are commercial preparations using bicarbonate of potash, instead, and these have the advantage of not putting sodium into your garden--more important to areas with high sodium content in their water, like the desert.