Preventing blackfly on broad beans

My Dad used to spray them with soapy water.

That's interesting.
I've heard that marigolds have the same effect, but never tried it myself. I think a trial run next year may be in order.
Where would I find draucus carrotta seeds?

Lots online - here’s one example
www.sarahraven.com/products/daucus-carota

Marigolds or French marigolds planted amongst them.
And spray with soapy water.

I bought mine as plug plants several years ago. Just a few. They are still doing well.
www.cumbriawildflowers.co.uk/habitats/wildcarrotdaucuscarotaplugplants

This was on Gardeners’ World last night. A viewer was filmed showing his way of preventing black fly by growing parsnips into their second year so the umbelliferous flowers can attract hover flies and other insects which feed on aphids.

I was intrigued because we allowed some of our parsnips to flower this year because we wanted to make a floral display of allotment wildlife attractors for a memorial service for one of our group members who has recently died.

The nearby bed of broad beans had no blackly at all whereas a few surplus plants grown at the bottom of the plot were covered in them.

Thanks for the tip about parsnips. I have found soapy water good, but needs frequent application.
My best tip is to plant in the autumn - they then come through before the blackfly have got moving - doesn't work if you want beans throughout the summer, but makes the best use of my small raised bed.