Hi all.

When i poured boiling water down the plughole, it simply drained away
without a problem. Thought this was strange so filled with hot water
from the tap which again got stuck.

This led me to boil up 3 large pans of water and bring to the boil. i
put in the plughole and filled the sink with boiling water, removed the
plug and it drained away easily. I then filled it with hot tap water
and it drained away fine a couple of times, then got stuck on the 3rd
time again.

Any ideas as to this strange behaviour??? I will try the caustic soda
this weekend.