I've just been out for another look. As I approached, but was still some distance away, I saw what I thought was a bumblebee in the vicinity of the flowers, but by the time I got closer, it had gone.
There are quite a few flowers on some of the plants, but I can see no sign of any small bean pods growing on the stalks where the flowers are no more. My dad always maintained that, when watering runner beans, it helped the flowers "set" by using a rose on the watering can and dowsing the entire plant. I do this, but I have no idea whether there is any scientific justification for so doing. I also give the plants a good dose of water around the base. We've had no measurable rain for weeks and this could be a factor.