This is very pretty, but sorry I can't help you, have never seen it before. Somebody who does know will be along shortly.

Flowers look like Thalictrum Hewitt's double?

Thalictrum delaveyi or aqualegifolia ?

Definitely not aquilegfolium, that finished flowering over a month ago . Now sure it is Thalictrum delaveyi Hewitt's Double. Have posted a photo of it flowering in my garden

Could be delavayi but not hewitts double as the flowers
Are not double

sorry to disagree - have a look at Spritzhenrys photos

Yes spritz has pics of hewitts double with double flowers,the pic in question has no double flowers!

I'd say thalictrum delavayi from the leaves.

Thank guys. Good to kow there's so much knowledge out there.