First Rule Of Plumbing

Spot the mistake :

The upstairs toilet has a blocked sink.
Remove and clean the gunk filled U-bend.
Replace U-bend and test sink. It is still blocked.
Remove pipe from U-bend to wall, find it is full of gunk.
Clean out pipe outside and return to upstairs toilet.
Hands are dirty, wash hands thoroughly in sink.
Mop up large quantities of water from floor.
Replace pipe.