why will water not drain from hand basin in bathroom?

Beyond the trap the water runs on - out of the house - in my bathroom it runs through an outside wall - if the pipe through the wall is too flat, still water with sludge in it (grease residue from soap mainly with skin flakes and hair) can collect and will block the pipe over time.

You may need to clean out the pipework the trap connects to - something flexible might be best - access from inside or outside whichever is easiest.

Good luck...

Peccavi
October 2009