This is greater celandine, Chelidonium majus, a member of the poppy family. Not to be confused with lesser celandine which is a member of the buttercup family.
These are mutations as they clearly have more than the requisite four petals. It seems they may be a Flore pleno variety, this means double flowered, although they look more triple flowered to me. They are said to bloom when the swallows arrive and fade when they leave, chelidṓn is Greek for swallow.
The entire plant is full of toxic alkoloids. This means that in small doses they probably have therapeutic uses as antimicrobials, analgesics and immune system stimulators. It was popular amongst herbalists of yore. Culpeper says of it;
“It is good in all old, filthy, corroding, creeping ulcers wheresoever, to stay their malignity of fretting and running, and to cause them to heal more speedily: the juice often applied to tetters, ring-worms, or other such like spreading cankers, will quickly heal them …”.
Tetters refers to any eruptive skin conditions and no doubt gives greater celandine its other name of tetterwort.
I would caution against rubbing any part of the plant on any part of the body as it apparently smells atrocious and readily causes contact dermatitis. Perhaps it is best admired on a sunny breezy day bobbing amongst the pretty pink of the herb Robert.