I was going to say get a cat as well, but that has already been suggested. There is nowhere on Earth which is rodent free, as mice have taken over the world. I once read that 25% of the world's food is either eaten by or contaminated by vermin, so let's all blame the price hike on that then, shall we?
By the way a mouse can fit through a 6mm gap, which is the same width as a BIC biro. Test all holes with your pen, and if that can get in, so can our furry little friends.