Do you have pollenating insects? Also where are you in the UK, we live in the north of Scotland and runners are pants so far as we are concerned.

I noticed my neighbour has a poor flower set this year, the dry air doesn't help,

We've tried growing runner beans (and so did our British predecessors in this house) several times, and although we got massive amounts of healthy leaves and flowers, there were virtually no beans. We were told (somewhat smugly) by our neighbours, that "English beans" find the summers here too hot and dry, and despite copious and assiduous watering, will never set seed. French beans do really well here, incidentally. As Pam suggests, it's probably the dryness of the air.