Mystery Flower

difficult angle but could be Hypericum hidcote

looks like it to me

Its Hypericum alright, but it ain't Hidcote at 8-12 inches high - it'll be Hypericum calycinum, probably.

Sorry missed the height bit. Calycinum can be invasive?

Sure can - I speak from experience, and not one I care to remember - back ached for weeks after clearing it from an area 20 feet by 20 feet....

Yup agree with Bamboo and on how invasive it is :-)

Thanks to everyone for this. Will remove and pot up to stop it overtaking the border.

Oh lord, didn't mean to put you off - you can always restrict its growth by digging it up occasionally and splitting any bits you don't want off the plant.