I suspect that it's come up from the flat below - they've had a new central heating boiler put in today, with lots of noise and drilling etc., maybe it disturbed a nest. Or maybe I've had them for a while and just haven't noticed.
Fortunately there isn't any accessible food, I hope - I tend to keep packets etc. in a wall cupboard, and right now the only food that isn't in there or the fridge are a few onions and potatoes and a bowl of apples, which I'd better cover.
Tomorrow I'll start off by cleaning the kitchen, then I suppose I'd better go out and buy some traps etc., and get used to checking them occasionally, and being more careful about crumbs etc.
It's not exactly the first time, this is a Victorian house so mice etc. sort of come with the territory, but it's a bit of a nuisance.
And before heliograph asks, I don't intend to let the snakes out to chase the mice. I don't think anyone in this country sells snake-traps...