Confidence: I'm as sure as I can be.
Notes: This is a robust medium sized dragonfly (Anisoptera), where immature males (see image) are yellow and black. When mature the yellow areas turn to the palest of greens tending to yellow except on the flanks of abdominal segment 8 & 9 which remain bright yellow.
Unique among British Anisoptera both sexes of this species eyes fail to meet across the upper surface of the head. The wings are clear, with the inner trailing edge of the hind wing, exhibiting a semicircular cut out and angular transition to the true trailing edge.