Dragonflies with eyes that meet on the top of their heads over a distinct length are all the Hawker-type dragonflies (Aeshnidae).
In the UK three genera comprise the Aeshnidae family: Mosaic Hawkers (Aeshna), Hairy Hawkers (Brachytron) and Emperors (Anax). There’s just one Hairy Hawker which visually resembles more closely the Mosiac Hawkers than do a couple of the so-called Mosaic Hawkers themselves – what I call the Brown Hawkers, which are much plainer).
So concentrating on the dorsal side of the abdominal segments, now choose between an individual looking mosaic-like (spots down the abdomen), emperor-like (heavy central line down the abdomen), and a “brown” hawker (very little marking on the abdomen).
Mosaic: paired spots
Emperor: heavy central line
Brown: plain or small flecks