A close up of a Common green lacewing, showing the dark red eyes that are usually only noticeable if you look very closely. I assume it's a Common green lacewing, as apparently "green lacewings" covers about 2000 species.

Fun facts about Green lacewings:

• The adults often overwinter in houses so you might see them at this time of year, though they are out and about all through the year.
• They often turn brown whilst hibernating. I always assumed those ones were dead or close to it so I've learned something new here.
• Their larvae are voracious predators of insect pests – apparently among the very best and are used commercially to keep aphids at bay.