The rat man is correct, I would usually just lft a few floorboards and put down a load of rat poison, if that isnt practical then I would drill 13mm holes and pour poison through holes, rats are easy enough to get shot of, just dont be tight with the poison. They should be gone within two weeks. check around outside to see if you can see anywhere they may be coming in, you will very often see no signs at all. if you see where they are gaining access, do not block it until the problem has been rectified and rats are dead.