...in the loft, you realise why they're called pests. That's what I bought this device for. I set it up wrongly to start with. Read the instructions! Attach the casing at a height of about ten inches (25cm) above floor level, pointed in the direction you think they're getting in. Set the repeller to ultrasonic - if you set it to the audible frequency, the little blighters ignore it. Then sit outside and watch them going in under the eaves, and come flying out again. The really don't like it! And it doesn't do them any physical harm, though they may require counselling afterwards.
It isn't actually cheap, but it worked for me... All I have to do now is block up the hole.