Mice

OP, one other thing to check is neighbors that are over generous feeding the birds. Mice and rats will move in and breed like mad if there is a good source of food. Nuts and grain spilt all around the bird table or excessive amounts of bird food on the ground will ensure your vermin get well feed and breed well.

Edit> For those that think the ultrasonic repellants work I have one about 6 foot away from and aimed directly at a giant bait box. I still need to regularly replace the poison bait blocks.

Ah the joys of owning a house previously owned by an enthusiastic but clueless DIYer. You'll be finding f*ckups for 15 or 20 years to come.

Not too long after we moved in I was taking up the crappy laminate floor in the box room, circular saw set just deep enough to cut through it. BANG and all electricity went off. Einstein had run an unfused spur off the back of a socket, between laminate and sub-floor using alarm cable. 230V 32 amps potentially going down a cable rated for perhaps 12V 500mA, in a position where any time anyone walked into that room they were wearing out the insulation on that cable.

We're still not really sure what is holding up the spiral staircase to the attic which they illegally used as a habitable room.

We didn't have mice. We had RATS. Under the kitchen floor due to some f*ckup in the extension construction many years before our ownership. One adventurous one thought it'd be fun to climb out the hole left for the cable to feed the fridge (after having a good gnaw at it) and have a look around. I sealed that up tightly with a piece of wood and, in conjunction with the below, no more rats.

Get poison. They love BASF Storm, most chemists have it or maybe a farm shop in your area. They love it so much they keep coming back, so you need to keep replenishing it. They take it home and stockpile it and share it around and then they all die. Our adventurous little fecker was taking two large lumps back a day but I carelessly allowed the supply to run out, the fecker then ran / jumped over a mouse trap beside the fridge and entered the kitchen proper, my other half was naturally not impressed.

This poison makes them die of thirst, they leave your house to seek water. Just make sure there isn't a water source under your floor like a drippy pipe.

A rat needs a gap about the size of a 1 or 2 euro coin (depending on size) to get in, but a mouse only needs a gap the width of a pencil.

I couldn't give a fiddler's. Are they worse than the ones which break the neck or decapitate? Do you feel sorry for all the COVID-19 viruses you killed with hand sanitizer? 😁

They dont work. Companies that made them got sued and they arent allowed to say they work anymore.

You only hear the mice scratching when they have just come into the house and are making the nest and thats even only if they are making the nest under your insulation on the actual ceiling slab. Once the nest is made you wont hear them again unless its more new arrivals making a new nest. And you will never hear them if they are not making it under the insulation anyway.

Best bet is battery traps in the attic at the start of a cold spell. Get them as soon as they appear. Once they breed you'll have many more.

No it won't bring them in from outside. I know ours come in anyway and I normally put bait in the roof when I hear the patter of tiny feet.

The idea of bait is to entice them to eat it. So in order to work it must attract them first and then kill them later.

I wouldn't be so sure. I've read that a mouse can travel up to 100ft in search of food at night and they have a keen sense of smell. Quite a wide area to kill all mice.

Well I'll post this from the internet

'Does mouse bait attract more mice? Do bait stations attract more mice? Yes, bait stations can draw more mice to the areas they are placed. Many times, this is a good way of gauging how extensive your infestation is, although it could draw more mice from other places than you expected'