blocked waste from sink?

Hi
I live in a converted flat and the fall from the kitchen sink to the sump is very shallow and occasionally blocks. I have used caustic soda before to release the blockages that occur, this time however the soda has solidified in the pipe blocking it completely. Can anyone let me know if this is permanent or will the water sitting on top of it work its way through, if not perhaps someone knows of a good remedy to clear the soda.
Getting to the pipes will mean lifting the laminate floor in a large part of the flat.