Can anyone identify this type of fly and advise on how to permanently get rid of them? They're about 2mm long, black or possibly very dark brown. They're appearing on the bathroom window in small numbers every day. They appear to be fixated by the window. If I flick one onto the sill they immediately jump back to the glass. They're very easy to squish, so not a major nuisance but I'd prefer not to see them every day.