HELP MOUSE!

Jasper (a neighbour's cat who comes in through our cat flap) has just bought us a present, except the present has escaped somewhere in our house. I have thrown the cat out the front door and locked the cat flap. The mouse was behind the computer desk looking quite happy considering his ordeal, so I went upstairs to ask Mr PE what I should do. Not much help from that direction as he is having a weekend lie in. When I came down the mouse had disappeared from behind the desk and I have absolutely no idea where the hell he is.
My worries are firstly that he is lying injured somewhere and needs my help, secondly (and this might follow on from the first if I don’t find him soon) that he is dead (and beginning to rot) somewhere. Finally that he is alive and now wants to live in my house and poo in my kitchen.
Can anyone help me here? Hello to any new visitors from Blog Explosion if you know the answers to any of these questions then feel free to dive right in. Where do mice like hiding? What do mice eat? How on earth can I get hold of him humanely and then when I do get hold of him what do I do with him if he is injured? If he is OK, where should I release him so that the cat doesn’t get him again? Can mice climb stairs or should my search be confined to the downstairs?
HELP!