Help please!

It's a Japanese anemone. Someone clever will probably be able to tell you which one. Lovely plants.

Yep, love them - it'll be in flower for ages now. I could look up the variety, but does it matter really? Cut it down at the end of the season when it's all died back, or remove the dead material next spring.

this looks like september charm.
I have this and along with several other named varieties as well as the species A huphensis.

Agree with Seaburngirl. You will need to enclose it a bit as it WILL travel by runners pretty fast once it gets established... Good, long-lasting flower to compensate!