I'd really suggest some sort of mouse killing/catching equipment to be priority if you haven't already - they breed (I'm sure I read they live for about a year and have on average 6 litters of 6 baby mice in that time), gnaw stuff (wires, pipes, woodwork, fabrics) and even worse, as Claire said, but somewhat more politely, they pee...everywhere, on everything! it spreads disease and it smells
Some harvest mice took it upon themselves to invade my parents' loft a couple of years back, and they (my parents - not the mice!) put down some poisoned grain stuff which oddly is blue, and for some reason doesn't result in a loft of dead mice (I can't remember why!!), I think it is fairly slow to act in poisoning them and i think generally it's not supposed to be a painful death for the mice. It wasn't very expensive at all (though sorry I can't remember how much) and was worth it to prevent the problem getting worse.
Alternatively, get a cat. Though I can't promise it will guarantee you mouse-free as mine used to regularly bring 'presents' home!