As a Pest Controller of 13 years, I love to see the odd rat pop his head up and pinch a few nuts or seeds from our bird table, there are fantastic little creatures. If you are seeing more than you are happy with you could just ensure that you keep the numbers down by regular baiting, you really don't need a pest controller to kill rats, they are so easy to control. Go to a good farm shop and buy "Neosorexa" its the best on the market, put plenty down in plastic bags and protect it from other wildlife and dogs, the rats will chew through the bags to get to the poison, if they aren't taking the poison, move it somewhere else after a couple of weeks. I also used to keep pet rats.