Hot, dry weather, at a guess. The other causes are sparrows eating the flowers, or poor pollination, but runner beans like cool, damp climates, so don't cope well with the excessive heat, specially at such a critical time.

Thanks Bamboo, you are probably right, but today I looked in some of the flowers and found pinhead sized hard cased beetles, which are probably having a beanfeast, Any idea what they are?

No idea - pea thrips are tiny little things, but not sure if they're hard like a beetle or not. These usually cause silvery patches on leaves and pods rather than sitting in the flowers though.