|> McGregors pollination handbook doesn't show all that great a yield
|> reduction on plots that bees were excluded from. My bet is poor
|> pollination from some other cause like rain or temperature.
It depends on the variety whether that is even plausible. They
are reliable in typical UK spring weather ....
More likely heat or drought, both of which can cause the varieties
grown here (i.e. as broad beans) to fail to set. Cold and wet do
cause setting failure here, but that does mean temperatures with
highs around 40 and/or rain that doesn't let up for more than an
hour or so at a time. They are partially self-fertile, but are
mainly pollinated by bumblebees here, which may be why miserable
conditions cause poor setting.
I can't speak for the "el ful" varieties.