Unknown weed

I've never seen this plant before, and suddenly it's blooming in three different locations at work. One "waste area" where a lot of clearweed (Pilea pumilagrows, one woody path edge recently invaded by garlic mustard, and one very woodsy area (though adjacent to what passes for a sidewalk in rural Lincoln).
Any guesses?

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That would explain why I've never seen it - it's one of those crazy New England things.
;p
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*feels proud*
Its very common in South East England, grows in profusion everywhere (well everywhere in semi shade and particularly my garden) Collins Wild Flowers field guide to Britain and Northern Europe identifies it as native or naturalised to Britain and Ireland.
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About it being considered native in the UK too... well I've found the USDA plants data base is a little more liberal than some other sources on what's native or not.
At least it's clearly not invasive.
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...but its pretty so I dont care.
:D