Your gonna laugh at this but the best bait i've found for catching rats is some Crispy Fried Onion which can be got in either Lidl or Aldi, cant remember which! It has a really strong smell and rats go mad for it! It comes in a tub and lasts for ages. Even works around chicken coops where the rats are feeding on grain and not bothering with other baits!
Keep that to yourself now, and don't tell a soul! :-)