Cucumbers

I have three beautiful cucumber plants, this is the third year I have grown them, in large pots in the greenhouse... they have been most successful before...
This year, the plants are just as good as previous plants, very bushy and healthy EXCEPT... the large bottom leaves have started to develop what look like mildew spots, just a few white round spots on the leaves, but a few more have appeared daily...  Can anyone please tell me if there is anything I can do to halt this.   
Greenhouse is unheated and top window is opened on warm days for ventilation.
Any help would be really welcome

Oh my!!!  I am blushing now    I have Johns books (both of them!)   Did not think to look it up... just went straight for the forum...
Thank you so much... will go and have a read now..