Courgette mould - is it fatal?

Have you actually got fluffy mould or just a grey mottleing on the leaves?
Any chance of a pic?

It sounds as if it might be powdery mildew - courgettes are very prone to this and it is due to the dry weather.  You need to make sure you water regularly.  Having said that, I had it on my courgettes last year as did most of the plots around me and I had a fantastic crop.

Have a look here http://chat.allotment-garden.org/index.php?topic=38177.msg454426#msg454426
about halway down the page is a pic of the grey mottleing on the leaves which is ok

Top pic and the lower leaves on the second definitely powdery mildew,you are unlucky this early in the year.If not many leaves are affected you could take them off,the milk and water is a preventative at the first signs really.The only thing you can do is water well(roots not on the leaves,feed and it will not affect your crop if the plants are established so you still stand a chance of getting a glut but your plants may fail prematurely.