I don't think it's a poppy. I took the pics a couple of days ago and now there are new flower buds forming in the axis (?) between the stem and the leaves. The buds themselves look like the buds of my perennial cornflowers but the leaves and general growth habit are totally different.
I will post pics when it flowers. It has self seeded - never been there before. I've had a few totally new volunteers this year which is odd.