Female Coenagrions can be a particular identification challenge since their body patterns are all but identical, for several. This is particularly true of the “terrible twins”, Azure Damselfly (C. puella) and Variable Damselfly (C. pulchellum). The only sure way to identify them is by the shape of the trailing edge of the pronotum, the plate covering the prothorax to which the front pair of legs is attached. The female pronotum, incidentally, is what the male grasps during copulation.
For the sake of comparison, I have included the female Common Blue (E. cyathigerum). Together these three constitute the species that the great majority of folk in the UK will encounter.