They will be Cluster flies, very little you can do to get rid of them to be honest without employing a pest controller, the loft will need to to be fumigated. If you do get a Pest Controller, ensure he guarantees his work as its not a cheap treatment and you don't want to be calling him back. I charge �65.00 for an average 3 bed house, as a guideline. If he uses a powder (unlikely) it will be very messy and will cover everything in the loft with a mildly toxic powder.
Good luck