It’s amazing what you see when you just stop and look, and look some more. This is a tiny plant bug which has no common name, but is called Heterotoma planicornis. They grow to just over 5mm long, and it was a wonder I saw it on a leaf of my crab apple tree. It moved so fast from leaf to leaf it was hard to keep track of, let alone photograph it. The wide and flattened 2nd antennal segment, darkish ground colour and green legs help in identifying the species.
The adults and nymphs feed on small insects as well as plant tissue.
The adults are seen July to October on a variety of plants and trees, especially nettles. It is abundant throughout most of Britain.