They'd only replace the entire unit, so I bought a new plastic gear from Overdrive in the US, removed and stripped the actuator only to discover there's nothing wrong with the plastic gear - it's perfect. I replaced it anyway, put the whole thing back together and as I expected, the fault is still there.
The exact symptoms are:
- Start car and there's no warning messages.
Drive around like a granny and there's no warning messages.
Once you get up to speed or drive normally the message appears.
Stop, turn the ignition off, give it a few minutes and start the car again and the messages have gone.