Common Blue Damselfly

Blue shoulder (antihumeral) stripes are wider than the black stripes and the side of the thorax does not have a short black line on it. Males are blue and have a club sign at the top of their abdomen. Females are blue or dull green and have a spur on the underside of the tip of their abdomen.

See - http://www.british-dragonflies.org.uk/home

Very helpful site