On returning from the ponds and heading towards my car, I noticed this large white butterfly circling the water soaring. It had some blackish markings on the wing tips. It was hard to get some pics - it was going very quickly. It was bigger than a Cabbage White and I couldn't work out what it could be. When I looked at the photos - maybe Tailed Emperor. But it barely flapped its wings at any stage and I'd forgotten what this species looked like in flight because I usually see it with wings closed!