A mouse in the house- help please

They can get through a hole the width of a small finger. We used to get them in a previous house because there was a nice little gap in between the original part of the [old] house and the extension. They'd end up in the leccy cupboard, but it also meant they travelled along the space in the bedroom walls to get there, and we could hear them through the night.
A mix of various things, traps etc, including that expanding foam in the cupboard, helped, but it's hard to prevent them coming in in the first place without a fair bit of effort.
We have plenty here, but they never get in the house. Too busy eating the bird food in the cages, as they have a nice little route up to those.

 

Weather you use humane or inhumane traps as Nigel says they can’t resist a bit of Christmas cake.
I am just the same .