We found we had rats not long after we had moved into our property. It wasnt a very nice feeling. They were in between the floor of the bedroom and the ceiling of our front room. We were battling with them for many years, sealing up any holes, using wire mesh, having cavity wall insulation put in (although it needed it, we thought it would hinder them being able to climb inside the cavities) etc etc. We finally thought we had got rid of them so we started to build an extension only to find that we hadnt. We were very upset. It wasnt until we removed our rear exterior wall that we found the way they were getting in. It was an unused pipe leading from the drains into the cavity of the walls!! Needless to say, the pipe was filled in and we havent heard pitter patter since. I know it doesnt help you in finding their route in, but it does show that although you think you have looked everywhere, there is a chance you havent. I wish anyone who has this problem lots of luck, as it is not a nice thing to have to deal with every day.