Buildersmate is probably the best person to advise on this, or Buenchico and Barmaid, but they may not spot this question under Business. I don't normally like to see questions posted in more than one place but in this case I wonder whether you might get a better response under Jobs & Education (or Law).
I can't answr everything but I can givea few pointers from my HR experience.
First, you do have a contract even if you weren't given one in writing, and the Handbook will form part of that.
The company can now ask you to sign a new contract provided they give enough notice, but they can't change it retrospectively.
They could argue you for over 18 months you initially seemed to accept that overtime wasn't payable for these hours.
Good luck. I hope Buildersmate sees this and can help you further.