You will usually find them in a loft, they gather in large clusters on warm walls and will be buzzing around a light or a torch.
They will sometimes be evident in the rest of the house, more so upstairs, you will find that you suddenly have more flies around then ever before (they look similar to a house fly but their body is brown and looks a little hairy)
In hot weather weather you will sometimes see them emerging from under the guttering from outside, sometimes you will see them all over the walls on the outside.
You can not treat them yourself, as special chemicals and application equipment are required.
Call in pest control, and make sure their work is guaranteed as it can be an expensive treatment.