Couple of feral cats should do the trick! Mind, you have to be careful - cats can be picky eaters and hunters. We've had a couple who were good ratters, but others who would only go for birds. Then there was Luna, who hunted only stray hot-dog sausages and feral KFC!

Failing that, a terrier or two (larger breeds like the Jack Russell) might help. They were bred to hunt rats, so it's pretty much part of their makeup!