IM(sadly extensive)E, once you've seen one running across the floor, you've got an infestation, and nothing will get rid of the little bastards. Moving house can help.
Our Islington house ended up smelling like a vagrant's armpit, mouse poo everywhere, you couldn't leave a bag of shopping out for half an hour without finding a bloody mouse in it. They woke me up by prancing round my room all night (underneath the sonic repeller thingy), ate my flapjack, chewed the oven cable so we had no oven, and crapped in the microwave and I still don't know how they even got in there.
They regarded humane traps as a a little rodent version of Challenge Annika, were perfectly capable of retrieving Mars Bar from a non-humane trap without getting caught, learned to love peppermint oil, danced the night away to the vibrations from the sonic repeller, and bred fast enough that the poison didn't have much effect on their incredibly fast-growing population.
The only thing that worked was glue-traps placed cunningly across their usual paths, but that really is cruel so I didn't like to use those.