Keeled Skimmer (Orthetrum coerulescens)
Whilst the male and female both have the same patterns along the body, a black central line with black between the segments the background colour in the female is olive green rather than the bright sky blue of the male. Just to add to the differences, the male has a tapered body whereas the female is much more slender and even along the entire length of the body.
Given those marked differences it would be easy to think that this was an Emerald Dragonfly or perhaps a darter. It just goes to show that nature is always able to catch the unprepared (like me) out!