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I have cornsalad in my garden (not flowering yet though) - it looks more like this than gromwell I think, but I could be wrong (though look at Encyclopaedia of life picture for cornsalad - about the ninth picture. I think gromwell is bristly-hairy (?) but cornsalad is smooth (mine is edible).
Thanks for the ID tip :) Yeh, this specimen was from cornwall, where plants tend to flower a bit earlier than elsewhere in the country!
I found some in my garden flowering today - only 5 flowers open, so it's only just started here in Warwickshire. I'll have to wait a bit until it's got ripe seeds, so I can check it isn't one of the other Valerianella's though.
Lat/Lng: 50.1687, -5.1255
OS grid ref: SW768345