They're black and smaller than house flies. We're getting a lot in the
garden, they seem to fly/hover aimlessly underneath the washing on the line
(like gnats), but never seem to land on anything.

Same indoors, they congregate in the middle of the room, sometimes a dozen
or two, just moving around underneath the ceiling fan, never seeming to land
on anything. They're coming in through an open door (although an almost
full-lenght curtain is across it, the breeze lifts it at times and we
seriously need the fresh air)

I'll try and get a photo later on, but does anyone have any ideas what they
are, and how to get rid of them? I'm getting tired zapping them with my
electronic fly-swat