Think that is Circea lutetiana, Enchanter's nightshade. In spite of the common name it is not a nightshade (Solanum) but a relative of evening primrose. A bit of a nuisance weed, it runs but is easy to pull out in soft soil but will break off at the root if the soil is hard. Like moist woodland conditions, flowers are delicate and pretty in a low key way.
Enchanters nightshade has opposite leaves. I've just taken this photo showing the flower too. As @Buttercupdays said, it can be persistent. I pull it up in borders but it's still there thirty years on!
@didyw I can ID it because I have it in my garden!
As I want to give a home to as many of our local wild plants as I can, I give it some leeway in suitable parts of the garden but pull it up in my borders. It keeps popping up in new places, but it doesn't cause any harm to other plants, so isn't a major problem.
Enchanters nightshade in our garden is all over the place. It is easy to pull out but once seeding then you carry the seeds around all over again. Yes we are very much wanting wildflowers in our garden but some do take over and this is one.
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As @Buttercupdays said, it can be persistent. I pull it up in borders but it's still there thirty years on!
In the sticks near Peterborough
When you don't even know who's in the team
S.Yorkshire/Derbyshire border