Caught be Zev yesterday, and let lose in the bedroom in the early hours. I managed to evict it at around 4am. Kai
caught something very similar a few years ago, so it's obviously common around here.
Both of them were very small mice, this one had a body which was at most 3cm long, although with surprisingly large ears for its size. It wasn't that bright, and only managed to evade capture because I either had nothing around to trap it with, or it wasn't on a flat enough surface to entirely cover, so managed to scamper free. It was only exceeded in stupidity by Zev, who failed to find it, and got bored to the extent that she went to sleep, leaving me to capture it!
It's been previously suggested that it's a wood mouse, which seems plausible, in comparison to the pictures in the Wikipedia article.