This small leaf-hopper dropped in through the window. It’s an Oncopsis species, either O. subangulata or O. flavicollis. Both are types of bug, small cousins of the cicadas you get in the Mediterranean and tropics; these two species both living off the sap in birch trees. The largest cicadas are 7 cm in length and some make enough noise that they can damage your hearing - luckily we have no such problem with this 0.5 cm UK version. Of the two species it might be, I lean towards O. subangulata due to its colouration, but as the species are very similar, very variable and often occur together it’s impossible for me to be certain.