We had the same a few years ago, under our shed. It was because I'd left a large sack of fishing groundbait in there. They chewed themselves a hole in the shed floor.
We called the council, they came out and set poison for the, looked like blue coloured grain.
It worked, but they died a horrid death.
One day I could hear a noise like a cat screaming/crying, I looked out of the window to see one of them just sitting there writhing in agony, not pleasant. I shot it with the air rifle to put it out of it's misery.
So I can't recommend poison I'm afraid.
Not sure how I'd deal with them again, maybe air rifle, maybe an instant death trap of some sort.
