I think just the scent of a natural predator like a cat or a dog will deter an infestation of mice. I used to live next door to a pub whose cellars were next to my living room, and when my mum's dog died (they were always visiting) there was no more doggy niffs about and we started to get the mice until we got a cat - he never caught any in the house but his presence alone was enough to deter the little beggars, he was also great at catching spiders too. Current lot of furry fiends merely "play" with them until they stop moving. I shouldn't worry too much about your cat beng a surrender monkey, his presence alone should keep the mice away, hopefully........