I just grew one very much like this one called "CornFlower" or "Bachelor's Button". It's in my butterfly garden.

It looks rather like a Campanula (Bellflower) to me. It certainly isn't any form of Centaurea.

It looks like some form of Centaurea; possibly Montana. The form doesn't agree with Campanula

No, it can't be, Bathgate. C. Montana buds are dark and spherical. When those buds open, we'll know what it is! LOL.

It is a campanula, looks like Campanula medium which is also known as Canterbury Bells - can be blue or white flowers, these look as if they may be white. I don't know where they come from but they keep popping up in our garden.

It's definitely Campanula.

Pity one cannot add a picture to an answer. We have Campanula plants with exactly the same buds as this.

Thanks everyone. Campanula it is. Looking forward to it opening out now. I'm always wary with 'unknowns' surprising me after allowing the last unknown to flower. (something called Lamb's Quarters). I'm still pulling it up 3 years later. Wish I'd pulled it up as soon as it appeared.

Actually, Sophie, I pull this one up as soon as it appears! Attractive, yes, but it seeds readily.

For the uninitiated Lamb's Quarters is also known as Goosefoot, Pigweed and Fat hen, ie Chenopodium album. At least it is an annual so if you stop it flowering it should not be too hard to keep under control.