[SM] That sounds very similar to the situation at my old flat; I tried traps and poisons, none of which worked. But then Dave came round and dispensed the hard stuff, and never a squeak after. But that was a modern flat, and filling the holes with expanding foam was straightforward. After our first visitation we called Dave in straight away, and it took him three attempts to get rid of the first batch; and expressed doubts that given the way the plumbing had been done in the kitchen that he could offer anything like a permanent solution.
I caught one of the little dears in a humane trap some weeks back, also using peanut butter. He died in it anyway, for reasons that aren't clear to me. He was only trapped overnight, perhaps six-eight hours, so it presumably wasn't hunger or thirst. A broken spirit, perhaps?
I've dealt with mice in two properties - each time by setting half a dozen traps at a time, baited with cheese. On each occasion they'd given up visiting within a week, and a dozen or so deaths. Perhaps the smell of mouse death lingers in the air? One bit of information I was given at the time was useful - a mouse can get through a hole the width of a pencil.