The pop-up plug in a hand wash basin is missing. (This assumes you want to replace it with a proper pop-up plug, not an ordinary plastic or rubber one.)
What you can do
- Measure the size in millimeters across the hole at its widest point.
- Buy a replacement pop-up plug at a plumbers’ merchant, such as the plumbing shop between the entrance to Burgess Park and the Trafalgar pub. (Near the parking shop)
- You may need to buy a whole plug/waste outlet assembly. The theory is that they are matched to be leak-proof. However, you may find that the plug fits without changing the assembly.
You can adjust the height of the plug by moving the screw at the bottom in or out.
To change the whole assembly or adjust the lever that pushes the plug up, see this question.