Hello justatwix, you have my sympathies! We've had problems in the past and I now how difficult it is to solve the problem. In the end we got a cat and that did the trick. You could always borrow one! Also, have a look at this:
I know this is a really old thread but just in case you haven't solved the problem and happen to see this then definitely definitely try blockmice.com I had a real problem with mice and am ridiculously, irrationally scared of them. They kept coming back and I spent a fortune to get people in to put poison down which never seemed to work. With block mice I had the full mouse-proofing service nearly 3 years ago now and I haven't had any problems since. It costs a bit more to start with but it works - they go through the entire house checking every possible entry point and blocking it. The best money I ever spent! Good luck!