Why are myflowers dropping on runner beans

Drought?

Mm. I had the same thought, Bamboo. Set your remaining and any subsequent flowers with a long daily mist spray of water, as well as keeping the plants themselves well watered.

That's exactally what I was going to say,
Lack of water ON the flowers.
Obviously don't do this while their is bright sun shine on them, I wait untill after 8 ish (PM)