Belong to the Ranunculaceae family only growing to approx 20cm at the most in Australia apparently they grow to 30cm in the UK. They just love a shady, moist position with plenty of leaf mould dug through the soil. You need to have them well marked in the Garden as when they are dormant in the Winter, you can easily dig them up.
The first photo is Anemonella thalictroides 'Shoaf's Double' we imported from Japan.
Anemonella thalictroides 'Double Green'
Anemonella thalictroides 'Double White' from Otto many years ago, it has a slight tinge of pink to the flowers.