Anyone know a common name for this?

Fumewort (and other names)

Amazingly its related to the poppy family.

That's it, thank you!
I just googled it and in fact several places offer it for sale, so perhaps I shouldn't have called it a weed - rather a wild flower. I'm happy for it to flourish here - it's happy in shady spots and goes on for months with a lovely splash of colour where it was bare before.

Yellow fumitory? Difficult to get rid of if you let if get too well established.

I would like to know too witzend. I have a large clump growing around the base of my birdbath. I assumed it was a weed. It's been there for a few years but it is pretty.