Hi there! Funny thing I just learned - what you call corgettes, we call Zucchini on this side of the pond ! See, I learn something new and interesting every day
Before I get cocky though - we are talking about the large squash plant, aren't we?
If it were indeed the mosaic virus affecting your plants, from what I have read there is no cure for a viral disease in plants. Removal of infected plants and preventing infection in the first place are the only solutions. Aphids are the main transmitter for the mosaic virus, so their control is important to prevent it from spreading to the rest of the garden.
However - if the only symptom you see is the small yellow blotches on the leaves, and it otherwise looks healthy, then I would suspect some other culprit. It could be several things, but the following link is the only one that I have had personal experience with on squash plants.
If you see no squash bugs or eggs, then it possibly could be a fungus or a disease. See if any of the following are more descriptive of your symptoms :