Often overlooked but a common area of ingress by mice into homes is ground level air vents. 6mm is all a mouse needs to find its way into your home,
Most air brick vent holes are 10mm, and some older ones are much larger, this gives mice a great opportunity to escape the cold and set up home. Fitting a weld mesh screen with a gauge of less than 6mm will prevent mice from entering though the air vents. There are several ways to fix screens over the vents, just remember do not block the holes completely, they are there for a reason.
Weld mesh is available on most online market places and from some DIY stores. Option one is to to buy a product called mouse mesh, the company produces vent covers in several sizes that fit most common air vent sizes. They are simple and quick to fit, but are the most expensive option out of the 3 detailed here.
Option 2 is to drill the 4 holes around the vent, fit Rawl plugs to the holes and then screw the mesh screen into place. This option is the most time consuming. Often the mortar around the vent will crumble which means getting a secure fit is difficult.
Option 3 is to simply adhere the mesh with silicon, this works well if you use a fine gauge mesh. Modern vents tend to have a recess which the mesh sits nicely into.