The most beautiful of all the damselflies is the appropriately named beautiful demoiselle (Calopteryx virgo). Sadly we don't see it this far north but I was lucky enough to see these earlier in the week in Northamptonshire. These are males (the second picture with flash).
Immature males are bluer with brown wings.
Females are iridescent green with gold wings.
Many damselfly and dragonfly species are spreading north with global warming so maybe one day will will see this beauty up here.