Thanks for the discussion, guys.
Sorry, I'm starting to feel a bit thick, but the force diagram as I see it is like this.
The vertical couple between translating force and anti-translating force rolls you.
As the CofG moves laterally with respect to the head, the horizontal couple formed between the two becomes a more powerful restoring force until the two couples balance.
If there was more vertical distance between the tail rotor and the main rotor hub, there'd be a greater rolling force.
Why is this wrong?