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These two species are pretty similar. Males in both species have tinted wings, and can be told apart relatively easily by the amount of pigmentation (C. splendens has a smaller, more distinct band). Females are less easy to seperate. You can't see the wings in this pic, so difficult to tell the sex for sure. One way to tell apart is by habitat - C. splendens prefers slower flowing water - C. virgo on faster flowing streams...
I think it must be a male, as there are curved appendages visible at the tip of the abdomen. I would expect the smaller, more distinct bands of colouring on the wings of C. splendens to be visible, even from this angle, and since they aren't I would tend to go for C. virgo on this one, but it's hard to be certain from this angle - if only we could see a slightly more side-on view!
I agree with the habitat tendencies for the two species, but there are places where they both occur, and I don't think habitat is sufficiently reliable to tell them apart.
Entomologist and biological recorder
As Martin says, the abdomenal appendages confirm that this is a male. I agree that it is not really possible to be certain which of the two species it is because of the angle of the view, but my opinion would be more on the side of C. splendens (there does appear to be a darker band on the wings in the correct position, with paler basal half + tip).
It's interesting how the body colour of both species can vary from green to blue depending on the angle of the light.
Lat/Lng: 52.9, -4.3
OS grid ref: SH4641