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It's either a Blue-tailed Damselfly (http://www.british-dragonflies.org.uk/?q...) or a Scarce Blue-tailed Damselfly (http://www.british-dragonflies.org.uk/sp...). Unfortunately, I can't tell the difference.
On the British Dragonfly society website it says that males have 'small patch of blue near the tip covering segments eight and nine'. The blue patch on this covered only the 8th segment. I didn't manage to get any photos with the tail in focus.