What does this pipe do? (Apart from allowing mice ingress)

[GrendelsMum]

A mouse has moved in (nights only) under my bedroom wardrobe. Although I sympathise with its desire for more comfortable night time accommodation, I think it could find that somewhere else.

I'm trying to work out how it's getting into the house at first floor level, and I think I identified the first hole that it was using, and blocked it up. Now the mouse is back again. The only thing that I can see that it might use is a mysterious pipe sticking out of the wall near a set of pipes which do the drains from the bathroom and loo. This pipe's about an inch wide, and just sticks out of the wall for about 2 inches and then ends. Any ideas what it might be for? Could it be a mouse conduit? And if not, where on earth is the mouse getting in?

p.s. cat sleeps in the bedroom and is entirely uninterested in the mouse.

I have no idea but I love you for using the word 'ingress'.

That's your overflow pipe is it not?

Yeah, that's your overflow pipe, don't block it up

What did you block the first whole up with? Wire wool is the best thing for blocking up mouse holes, unless it that (or concrete or something) the mouse could have opened it up again...

whole? WHOLE? My goodness

The wisdom of Mumsnet triumphs again. I knew the pipe was for something useful!

Can I push wire wool into the overflow pipe to keep the mouse out? Can I do it temporarily? The first mouse hole was around some pipes (shoddy builders) so I polyfilla-ed round the pipes, and the mouse seemed baffled for a while. (We could hear it bouncing up and down on the pipe trying to get in.)

Mice can get through incredibly small holes, I hate to tell you. They can get through the holes in airbricks, for example. I'm afraid we resorted to poison for ours..

I think you can get a flep type thing to fit on an overflow pipe. It's just like a little hinged lid. It will flap open to let water out, but it should prevent a mouse getting in. Unless it's a very clever mouse