A kind person on Twitter identified it as Lunaria annua, whose English name is Honesty. There's a good article from the Torygraph, here. It says, "Lunaria annua will cope in most situations and seems happiest growing among other plants", which suggests it could do well in the understory of the hedgerow. And the seed heads look nice in a vase, apparently.
It shows, when rehabilitating an allotment, don't be too ruthless with the "weeds" until you know what they are. I mean, the nettles you can see round the Honesty, are just a bloody nuisance. But this plant is lovely. Another reason for not using glyphosate.