Common blue damselfly (Enallagma cyathigerum).
Mature males like this are blue, while females can take on other colours, mostly green. They are, as the name suggests, very common in the UK.
Once mated, a female lays eggs on plant stems, both above and below the water, while the male stands guard. The subsequent larvae feed on other invertebrates, then climbs up a plant stem where it breaks out of its chrysalis, and flies off.
These seem pretty sensitive to any movement during the day so it took me a while to get a decent photo!
Taken with a Nikon D3200 and Tamron 90mm.