I wouldn't recommend poison for two reasons: it's neither humane or wise.
They die a terrible agonising death from poison and as they don't want to display their vulnerability to predators as they're dying, they often crawl in to really awkward places which sod's law will be under the floorboards, meaning that you'll have to take the floor boards up to take the dead rat away. This leads to another sod's law...you can guarantee that the floorboards won't lay back down properly, particularly if it's a very old house!