Why have I so few runner beans but plenty of blossom leaving short stalks?

If you mean that the blossoms are falling without leaving juvenile beans behind them, then your bean flowers haven't been pollinated. There don't seem to have been many bees about this year to do the job.
If you still have flowers, mist them twice a day, even if it rains. The fine spray of the mister will help to set the beans.

I have also noticed the bluetits in among them and leaving a trail of petals on the path.

are you sure that the short stalks couldn't be just the very start of the bean?
I'll apologise in advance if you've grown them before and are only shaking your head at this comment!