You will catch them with a smear of peanut butter in one of these:
http://www.pestcontroldirect.co.uk/a...ftincat%2ehtml
...then drive to work or somewhere and let them out miles from home, in the knowledge that Linda McCartney is smiling down from heaven upon your humanity.
It will be a battle of man versus beast to figure out where they are coming in, they are very ingenious, and apparently can fit through any hole big enough to poke a pencil into. When you find it, you need to fill the gap up with that expanding foam in a can, apparently mice never chew through that (at least we've kept them out for years with it). If you like you can make double sure with one of the ultrasonic/electromagnetic plug-in repellers from the same site above.
You don't have an infestation - if you did, you would be finding shedloads of droppings, not just the odd one - so won't need rentokil or poison. I think they just like to find somewhere warm once the cold starts to set in, but between the foam and the repeller they can be easily encouraged to go next door instead

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