Keeled Skimmer (Orthetrum coerulescens)
Easily dismissed at first sight as a Broad-bodied Chaser because of its bright blue body (the male that is) closer examination, if it settles, will show a narrower body tapering to a point at the tail. It also lacks the yellow side markings on the main body too.
The Keeled Skimmer tends to fly low over the water of its chosen pond, skimming over it in fact. It has a rapid and unpredictable flight pattern, usually near the edge of the water, where the pond side vegetation grows. The males settle on a favoured perch to monitor activity in their patch but show 'interest' in intruders rather than out right confrontation like the Broad-bodied Chaser.
The females are green and have a thin, less tapered body.