I just pulled out the washing machine the old owners "kindly" left us to find a hole where the wall meets the floor.
It's approximately 80mm x 50mm and goes down approx 100mm - below ground level - where it seems to open up again. There is a drain near it (on the right of the photo) so I assume this void is due to that. I can feel a breeze when I put my hand near it. Before I put in the new washing machine, I'd like to patch the wall to improve insulation and prevent mice getting in. What's the best way to do this? The wall is brick.
Edit: Thanks, Bib! Photo of the patch added: