Could they be getting damaged by wind? Or they may just be falling off without being pollinated - do you have insects around that could pollinate?

could also be too dry.

Runner beans like a tremendous amount of water. Also water the whole plant (not just the base) as this aids pollination. Give them a good shower every day after the sun has gone off them.

Thanks for help but the beans are fairly sheltered and there are loads of insects I have tried planting them in the garden and containers with a good mulch of shredded paper to absorb the water and keep them moist but it is always the same. This year I have tried grow bags and until last week the flowers were fine and then all of a sudden they just snapped off. I will try watering them more often and hope it works.