There are two hawkers, technically classified as Mosaic Hawkers (i.e. Aeshna), which don’t really look mosaic (paired spots down the abdomen).
Both are essentially brown or tan in colour. One is rich brown in colour with paired small flecks on the dorsal side of abdominal segments; there is a single pair of spots on S2. The other is more tan coloured and quite plain save for a thin dark central line and its eponymous isosceles triangle mark on S2.
Brown Hawker: paired spots on S2
Norfolk/Green-eyed Hawker: triangle mark on S2
Being hawkers, these two are often reticent to settle.
There is at least one identification feature, particularly useful for distinguishing these two hawkers in flight, that does benefit from a colour approach, and that is the so-called “tobacco tint” present on the wings of the Brown Hawker (Aeshna grandis); this can often be seen in flight.
The other suspect, the Norfolk Hawker/Green-eyed Hawker (Aeshna isoceles), has bright green eyes but only when mature; immature specimens do not have green eyes at all. If you do see a “Brown” Hawker with green eyes, though, then Norfolk Hawker it is.
Brown Hawker (Aeshna grandis)
Norfolk Hawker/Green-eyed Hawker (Aeshna isoceles)