Apparently we did a good job of flushing out the ones in the kitchen with our trap - three in the end, and none for nearly a week. Removing the panel under the units, Dave remarked "Missed one" and picked up a cardboard mouse by its tail. At least the source of the slightly musty smell has now been explained.
It was quite odd - I said: "my wife saw running feet behind the skirting board there", pointing to a wall in the bedroom. Dave immediately went into the bathroom and fumbled around the pipework. I'm sure this guy must have been a mouse in a previous existence...