A couple of summers back we had a rat come into our house and make a home for himself, and called these guys:
http://www.environpestcontrol.co.uk/the ... /about-us/. Amazing - we haven't seen a rat inside since. They come with a dog who sniffs out where the rats are actually moving through your property, allowing them to bait efficiently and find the holes where they are getting in. They can also help seal up the holes as part of the service.
Just also worth noting that it's important to be careful when putting poison down yourself outside. Even if you don't have young children who might want to explore what it is, you should be careful about poisoning any local pets.
I wish you the best of luck getting rid of your unwanted guests!