It's a hybrid tree peony, a mixture between a tree peony (which you don't prune) and a herbaceous peony which dies back in winter. Cut back to about 25cm in the autumn and new shoots will grow from the tips of the old shoots and also from the base each year. I've got two which are about half the size of yours and they are five years old, I've only ever had a few flowers but more each year, they are beautiful!
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Gardening in Central Norfolk on improved gritty moraine over chalk ... free-draining.