Just before going to bed that night I left half a dozen small squares of bread and marmalade on a plate on the floor. I didn't know if mice like bread and marmalade but I figured that if there was a mouse in residence it might be grateful for a late night snack, and if the bread disappeared at least it would prove that I was neither imagining things nor losing my marbles - and sure enough, when I checked the following morning all but one of the pieces had disappeared.
Now as much as I like mice, and having one in residence didn't particularly bother me, it wasn't an ideal situation - where there's one there could be twenty one so the mouse had to go, and the sooner the better. But I also had to find out where it had got in, and as it seemed to be frequenting the corner behind the computer unit that was the obvious place to start; not an easy task though in view of everything which had to be moved so it would have to wait until the weekend when I had plenty of time. So Sunday morning saw me moving all the books and various boxes of computer accessories, paper, envelopes, laminating sheets, files and plastic pockets from the shelves in the unit, disconnecting and moving two computers, two printers, two monitors and a scanner, and finally moving the unit well away from the wall. It took quite a while but eventually I could see where my little visitor was coming and going; right in the corner was a gap between the floorboards and skirting board - not a particularly big gap, but certainly enough for a mouse to get through.
So after a walk down to the local DIY store I returned with a can of expanding foam which I sprayed into the gap to fill it and seal it, then once the foam had hardened I cut off the excess and made a start on putting back the pc unit and all its contents. I wasn't sure if the mouse might still have been hiding somewhere else in the room though; if it was I would need to get a humane trap, but before going to the expense of buying one if I didn't need to I decided to leave some more bread and marmalade down just to be sure. I did that twice and on neither occasion was it touched, so I think the mouse must have evicted itself before I sealed up the hole. There's certainly been no more strange noises or any other evidence of it still being around so I think I can consider that my efforts on Sunday, although time-consuming, have definitely been a success and my uninvited guest has gone for good.