(non-trade)

I'm guessing one option is to let them know in a non-threatening way that you're prepared to quit and go phase 6 by yourself if they won't let you, which sounds like a lose-lose situation as they'll still be down someone in the new year.

Is there any way to make the situation work for both parties? If they let you go for phase 6 and don't have you for the new year, what can you do to make things easier for them? Or is there something they can do for you instead of letting you go for phase 6? Immediate pay rise or similar?

Get any promises of future benefits in writing, or better yet no promises, just immediate actions.