I've seen these insects before (here in Cambridge, UK) and thought that they were probably mosquitoes, but they don't make the annoying noise that mosquitoes do (they fly similarly, but silently):
I also noticed that they are perhaps slightly larger than mosquitoes (maybe 12mm long) and don't have the long mouth which mosquitoes use to sting you.
Sorry for the poor photos, I don't have a camera with a macro lens anymore, and it's night-time. It's casting a strong shadow as you can see in the second photo.