Black Fly problem

Does anybody have any ideas of how to get rid of black fly on broad beans?

The best control for all aphids I've found is Permethrin, a contact toxin. Although not poisonous to mammals (other than cats) it is harmful to other insects just after freshly spraying. But it dries off quickly and will only then kill sap sucking insects, so pollinators are unaffected. In any case you should be directing spray at foliage and not flowers. Fruit and Veg sprayed can be safely eaten 24 hours later.

Unfortunately you can't buy Permethrin at garden centres any longer because of an EU ban, but it's still available from Amazon. But it needs to be diluted down by about 6:1.

If I remember correctly it was sold under the trade name of picket !

It did use to be widely available MUMS but was banned around 10 yrs ago 

Prevention is the best way. Soapy water or hosed jets is the only answer. But it has to be done before the ants colonise a plant big time ... at the first sign, and then at regular intervals. Apart from that, plant earlier, preferably in the Autumn. The early planting works a treat.