So, I did a bit of internet research and found an interesting and easy to make recipe for a mint and garlic spray.
Apparently, blackfly hate the smell of this concoction and give any plants treated with it a wide berth.
I made it up over night and gave all the runner beans a good drenching with it in the morning.
So far so good. The plant that had loads of black fly on it still looks pretty sickly but the others don't seemed to have become a target.