The problem I found, as you may have, is you hold the mouse by its tail, by the time you have aimed the hammer and dealt the blow, the mouse has moved. For some reason, under a plank they don't move about. On the other hand you may have come down from the loft covered in blood. I cornered a mouse once in an old house, and kicked it in the head with my steel toe cap. It came out like a fountain about three feet high.