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There are various colour forms of the female of this species and the typica form resembles the markings of the male.
The best way to distinguish between the sexes in this species is that females have a backward pointing spine on the underside of the abdomen which males lack. Males have two small appendages on the end of the abdomen that are visible when looked at from above.
The individual in the picture appears to have a spine, indicating that it is a female but this is not sufficiently clear in the photo to be 100% certain.
This is a male.
The aparent 'spine' at the end of the abdomen is one of the two appendages that Jonathan corectly says would be visible from above on a male (these are seperated in the male, two smaller appendages that are much closer together are also visible in the female). The spine on a female is small and difficult to see (+ not right at the end of the abdomen), the much larger ovipositor is far more obvious.
In addition the shape of the blue marking on the side of the ninth abdomen segment points to it being a male (the eigth segment is the one that is blue on top, the ninth is just behind that - towards the tail end). In males the border between the blue and black resembles a 'tilde' marking (~ http://en.wikipedia.org/wiki/Tilde ), in females the border is more like an inverted 'U' shape - although not in field guides this does seem to hold true (compare with the female in this observation: http://www.ispot.org.uk/node/64913 )
for the detail. I can see the tilde border so agree it is a male.
Howardian Local Nature Reserve
Lat/Lng: 51.4, -3.2
OS grid ref: ST2078