A simple thing you can do, if possible is get up there often, make noise and move stuff around. If you use the attic for storage then clear out as much as you can. I have the same problem in my garage and the moving stuff making noise strategy keeps them at bay. Yes get a specialist in but if the environment is what rats need, shelter, safety, food nearby. then new rats will continue to move in...and yes see if you can find how they are getting in and block it. As others have said, be careful its not bats who are protected. A webcam might be useful to find out what is going on before you act if you can't get a specialist in.