Very usefull reading, a mouse and possibly friends have arrived under our house. I have followed the collective advice and laid some poison bait trays down where we found droppings and where items have been chewed. We have 2 air bricks under the house with holes about 10mm square to allow air to flow and keep in dry. I can only imagine they either climbed through these or they got in when the door was open!
Any advice on how small a gap is needed to stop mice would be appreciated, as I will fit smaller wire screens if required to block this route!