Big Butterfly

We were just arriving home this afternoon when I spotted this enormous butterfly on the geraniums beside the back door.
It posed for ages and then fluttered off to sit in another bush for a while.
I don't think I've ever seen one this big in the garden. It must have had at least a 10cm  (4") wing span.

Helped out at the Revue tonight and will enjoy watching it properly from the audience tomorrow.