I would point out to Philip Milton that perhaps NHS Trade Union leaders remember the words of Martin Niemoller: "When Hitler attacked the Jews, I was not a Jew and did nothing. When Hitler attacked me there was no one left to be concerned."
They may remember that before the election David Cameron, with an honesty rare among politicians, said that his promises were cast iron. He used those very words when promising a referendum on Europe.
As my Oxford dictionary makes clear: "Cast iron is relatively brittle."