It could just be passing through looking for a new territory (population growth or competing animals like rats will push them to seek a new home).
If so, Little Nipper traps work very well and are sold by the hardware store opposite Tesco on Southwark Park Rd at the Blue (and certainly elsewhere) for about £1 so get several. These traps have a hair-trigger so be brave or get someone else to set it. Use a tiny piece of toast with a dab of peanut butter (easy to attach to the spike and has an attractive aroma), and point the traps at right angles towards the skirting board, so the mouse will have to cross the baited end of the trap to follow the skirting board around the room. In the morning you should have caught it.
Sealing entry points such as beneath doors, or around pipework, and most importantly removing food sources (hoover and mop frequently, put food in sealed plastic containers rather than cardboard cartons, & don't forget to close the food waste box tightly) can make your home a relatively unappealing place to re-establish.
If the mice are well-established and you have just noticed, several days trapping will show this. You should then take the preventative steps above, but also call a pest controller who will probably lay some poison bait traps.
The poisons sold to the public are worse than useless because the reproductive rate of the mouse population means they will quickly select for resistance to poison, and as everyone uses the same poison over and over, the resistant population is growing. Continue to lay this poison and you are effectively leaving mouse food out. Licensed pest controllers have access to more potent poisons which are then removed when they have done their job.
A cat can help if you have the right one, but correspondingly their food bowls will attract vermin too.
You may be lucky if it is just the odd one, but unlikely.
My mother had mice for a while when there was building work next to her house. (Must have disturbed a nest)
She tried everything, blocked up any gaps in the walls, used the plug in things that let out a frequency, traps, she would literally pull out the fridge/cooker etc every day to make sure there were no crumbs but nothing worked. Till we got a cat.
if you are not too keen on the idea of taking on a pet, but if you can borrow a cat for a couple of weeks or even get hold of some used kitty litter (disgusting i know) the scent should keep them away.
Check the obvious places that they get in. Behind the toilet and under the sink, if there are any holes plug them up. Think everyone in London has this problem from time to time, don't worry too much and make sure no crumbs are left around kitchen.
I have lived in the same flat for 17 years and suddenly last year we started having mice and my neighbours confirmed they also started having them. Completely out of the blue, no works nearby. I tried so many kinds of poisons, human traps, peppermint oil and also the traditional trigger traps, nothing worked. I spent hundreds. Until I got the glue traps. I was not keen on these but they turned out to be very effective (the gruesome bit is that you have to actually kill them). However, after catching a few, they got the hint and I actually saw one jumping over 2 rows of them. They are very clever little creatures. The whole flat was booby trapped and they still managed to run around. There was not even food around, I had packed everything in containers. One weekend in January, after not being able to sleep, again, I was on a mission. I was crazy. I closed all entries with expansive foam and steel wool (they can squeeze through a hole the size of a pencil). You should have seen the sight of me, my hands covered in that foam and I had to run to B&Q late Sunday to find a solution to remove the hardened foam (actually always wear gloves, there is no magic removal product). On that evening, I had a business meeting with a Japanese client, I was so ashamed with the state of me!! But hurray, I have not seen another mouse since. I can still hear movement in the floorboards and although I left traps/poison there, none were caught so far. They must have a 6th sense.
Get a cat! In many years of living in Victorian Houses we never had any living in the house, occasional dead one brought in through the cat flap. I am now in a flat but still have my cat to keep me safe!
Been reading SE1 forum for years, lived in the Borough for nearly 50 years, I can't quite think why the mouse post moved me to join up. Still: "Karen", house mice are house mice and country (garden?) mice are country mice, they can't live in each other's territories, it's not rescuing the wee sleekit etc. beastie to let it go outside.(on reflection Burns's beastie was a harvest mouse). I've found that a bit of Bournville choc and a Little Nipper always (well, twice so far) solved the problem.
A friend of mine saw a mouse in his flat and shouted at it "BAD MOUSE!". Weeks later he found it dead under his sofa. Ok, that's not helpful.
Spring traps are the most efficient,humane way of getting rid of the little bleeders - poison is slow and they tend to die in the most inaccessible places.
I would always recommend getting a cat as I really like them!Mind you, when I lived in an old house with mmy cat Buki, the little sod would half-kill them then let them run around a bit. She seemed to think my screaming was my way of saying thank you!