The fourth floor flat we recently moved into has a very slow draining sink. The sink trap and the initial plastic pipe is clear. A plumber we had doing another job pumped some air into the pipe with minimal effect he said that a blockage/gunge must be building up where the plastic pipe joins to a metal pipe that joins the communal down pipe in the pantry. To get to where the pipes connect he would have to take out the unit next to the sink and I want to avoid that. Is there anything safe to pour down the sink that will reach that part to clear it? George.