Though I had never seen one of these before, with those flattened parts of its antennae there was no mistaking this little insect. This is Heterotoma planicornis, a tiny member of the Miridae family of plant bugs. It can be found throughout Britain, on various plant species – mine was on the leaf of an Alder tree. The NatureSpot website reports that the eggs hatch in May and the nymphs develop in to adults from June onwards. The tiny bug I found was still a nymph so I guess it will be making its final change fairly soon.