The old gasket made of black plastic snapped when I tightened the bolts to the base of the toilet so I bought a replacement flange to find that the old one has been cemented/glued in? Removed the lip but cannot prise out the tube going down the waste pipe to put the new one in. Should I try to saw or chisel it out and how do I know how much to remove of the black plastic as it seems to be a long continuous pipe? Help!!!!