all my french marrigolds have started to get white patches on the leaves can anyone advise what is causing it please

Sounds like powdery mildew. This could be due to too little watering, too much watering or putting them too close together so that air cannot circulate between the plants. Pull out the ones with powdery mildew, and burn them. If you put them on a compost heap, the problem will recurr. The remaining plants might be OK if you catch it early enough. Alternatively, start again.