I have a pop up waste (controlled as usual by a rod) on a bathroom basin that I was hoping to change to a sprung waste, cos the pop up thingie no longer works.

BUT having removed the rod and basin trap I find that there is no obvious way of unscrewing the basin waste.

Basically there is a screw thread that the trap attaches to, but nothing but a smooth piece of metal from there into the basin. Nothing to attach a wrench to or that it could grip.

What to do?

Here's a photo:

http://i1376.photobucket.com/albums/...psxqay0ypy.jpg