This chap crawled out of our pond a few days ago along with a few friends. Apparently they are Southern Hawkers (Aeshna cyanea).
We've been finding nymphs in the pond for the past couple of years but this is the first time I've seen them emerge and worked out what they are. It also explains where our tadpoles are going.
I watched this one crack its larval casing and inflate its wings over the course of a couple of hours before flying off. Well worth the price of a few tadpoles.