A common darter dragonfly (Sympetrum striolatum). This one was resting on this plant stalk overlooking the water and whenever another common darter came within 3 or 4 metres, it would fly up to chase, intruder off, and then return to exactly the same spot to rest. It must have been guarding this stretch of water from rival males. Yesterday I saw a pair mating on the wing, repeatedly dropping down and dipping in the water, presumably laying newly fertilized eggs.
Photo taken in Reading University grounds, Reading, UK, on 2008-08-30.