A beautiful butterfly in flight..or is it a moth?!

....This butterfly/moth was about 12cm x 12cm, one of the biggest I've ever seen other than in a santuary, and quite beautiful.
....I was in a garden centre and saw this, buttermoth and followed it around for a while until it briefly settled, and being a saddo who happenened to have thier camera with them, whipped it out and managed a few shots before the buttermoth bored with me fluttered off for some peace and quiet!
....If anybody in blipland can help me identify this beautiful creature, please do!
Thanks for stopping by and I hope you enjoy a fab weekend : ) X