The Black Darter (Sympetrum danae) is a scarce dragonfly in Norfolk, but occurs in good numbers where it is found. As a species it occurs from North America to Japan.
They're black! At least, the males are - they're the only black darter in England and ID is not a problem. Females are best identified by observing them mating with a black male, but have a distinctive black triangle on the top of the thorax, and three yellow marks in the black band on the side of the thorax. Note also the all black legs, unlike the Common Darter which has cream stripes.
Found breeding only around Kings Lynn, but vagrant males do very occasionally turn up elsewhere. If you want to see them, go to Royden Common, where they are the most common darter.