Shuttle Shaped Dart (Agrotis exclamationis)
It is one of our commonest species, widespread and common across the whole of southern England and so readily found here in Dorset.
These little chaps, they are only half an inch or so long, can be found almost any time from April to October. They have several overlapping broods a year and so they can turn up in a moth trap at almost any time (and frequently do).
The larvae feed on docks, Dandelion, Knotgrass, Lettuce, and many other plants as well which is why they are so common.