What's this plant called please?

Macleaya cordata probably. Now called M. x kewensis.

That's it! Thanks Owdboggy - I'm obviously getting old... I just COULDN'T think of it! It was driving me mad. :-(

Yes, aka plume poppy.

I knows the feeling well. I can usually remember the name of a plant until someone says "What is that?"

Very invasive ,it is too and difficult to eradicate but it is very nice as a backdrop in a large border and I think flower arrangers like it.

So invasive that we have failed utterly to establish it anywhere we have tried to plant it.

OOOH! I've bought two of them for the back of my wide border. I hope they don't run everywhere I don't want them and just behave themselves and grow where I want them to... :-(

Weird, owdboggy, that they haven't established! Mine spread by seed & root...I cut off the flowers before the seeds drop. Despite that, they have spread over 2 years by about 1m each way - I pulled up smallish stems to re-pot & sell at the school summer fair, got about 8 pots-worth. Cut off the flowerheads this year to put in a vase - looked great for a few days but spread seeds all over the room! Don't think it can be that hard to eradicate, because you can identify the new seedlings/plantlets v easily & pull them out