"As often happens in difficult cases, two well established and valuable legal principles collide. One is, that it is desirable that Courts and Tribunals should have all the available material before them with which to arrive at a just conclusion in accordance with law. The other is, that it is desirable that parties should be in a position freely to negotiate a compromise of their dispute without having what they say in the course of those negotiations revealed subsequently and used against them in litigation or proceedings before a tribunal"