For stoats we set a Fenn trap, building a tunnel with bricks, and a woodtunnel the gamekeeper gave us. Since then caught mink and rats. Set along the wall side where the rats run, leave baited but not set till food goes regularly, then set, should catch fairly quickly. Also the giant 'mousetrap' type again put blocks round so innocent critters don't get caught.
Another idea I read was to leave bran in a bucket, when they've got used to going in for it, put water in and float bran on top.
Hopefully our cat keeps them down, occasionally a big one moves in, cat doesn't like going in barn.