Any question answered...

You'd be amazed at how small a hole a mouse can get through- at quite high speed.

When I worked in the All Bar One on Regent Street the mice could jump through the air bricks in order to travel from room to room, with no appreciable drop in pace.

They may not look like it, but they can happily go through a hole the diameter of a standard biro.

That's absolutely right--I have seen them go through such holes!

For commercial premises or a whole house, it'll be very difficult to make it mouse-proof, but a flat should be possible.