If you do leave, ask for an exit interview, and then politely, cogently and unemotionally tell them what's wrong. They'll probably agree with you. They've messed you around and (if you leave) they need to know it. In fact I bet you have a whole internal monologue already subconsciously prepared!
It would be great to stay and turn it all around (as suggested up thread), but are they/is it worth it?
BTW I'm afraid buggi's suggestion about tribunals may not apply to you as you haven't been there long enough (am prepared to be corrected on this). A shame as I would share buggi's glee...
And one question nobody's asked yet - do you have a union to turn to, or could you join one?