My first thought (how do these things pop into one’s head?) was bistort, but then it plainly wasn’t. I looked in a wildflower book and the nearest were some mint varieties – but it wasn’t any of them.
Then I asked on Twitter and Facebook, and on Twitter the answer came from Martin (@botanicalmartin) who said:
I think it’s Phytolacca polyandra or Chinese Pokeweed which I guess is a garden plant and not very common escaped in the wild – map here
I looked up the flower in Google and found some good photos – and it’s definitely Chinese Pokeweed. And the map that Martin mentioned also tallies because it show some specimens in Cambridge, which is where I photographed them.
They are on the fringes of an area of wild flowers in a park just a few yards from the High Street.
Were they sown, along with the wild flowers? Were they there from an earlier planting?
Martin has a WP blog The Intermingled Pot, if you would like to take a look at his take on nature and the state of nature in the UK.