If you live in London they're living just below you in the drains / sewers.
I remember when I was a child, my cat killed a family of rats, on by one, after they found a way up from the sewers through nearby disused outside toilet (used as a shed) and nested in our garden.
He was a big hunter. I don't think other cats I've had would have been able to take them in, they were massive.
The outside toilet was blocked off, problem solved.
More recently, rats got into the floorboards between my flat and the one above. The ratcatcher was useless, he just killed them meaning they died and stank out the flat above, but more came.
My plumber solved it by finding out how they were getting in the house and blocking it off - he lifted a manhole cover in my garden and we saw (look away if you're eating breakfast!) the waste from neighbours running along towards the sewer - a poo rushed past underneath us, then a rat scurrying along. My plumber said that was normal, they're always there, just below our feet in the sewage system, the challenge was finding out how they were getting in the house, often it was pipes left open but unused after dodgy conversions although he had heard of rats swimming up a u bend and popping up in a toilet!!! 
Sure enough you could see one of the pipes running from my house into this channel wasn't in use (you could see no water was running through it as it had loads of gunk in it) and the rats were using that literally as a rat-run into the building.
They must have always been able to do it, since the house was converted into flats, but as they couldn't access the house, only get to in between the floors, it wasn't any interest to them till I'd installed spotlights in the kitchen. And that provided heat. They were coming in to nest in the warmth.
It took about a year of repeated visits from various rat catchers and rats repeatedly chewing through my electric cables before I asked my plumber to have a look and he solved in it 10 minutes by finding the pipe responsible and capping it - no more rats.
If it's just a one off or if you live really and have rats living in fields I'd not worry much.
But if you're living in a town/city and have a rat run up.from the sewers somewhere near you, you need to find and block it IMO else more may come up.