I would suggest setting motor idle speed to Low in Naza assistant, and watch to make sure the props are spinning the right direction. If that's correct i suggest an advanced IMU calibration, and take off in GPS mode, not atti - experience I just had which caused a flip.
It works now! Thanks for all replies.
The problem went away when I did an advanced calibration. As I said, it was calibrated basic the day before. Today when I checked the Naza assistant, there was a warning saying an advanced calibration was necessary. So i did, and then it worked fine.
In GPS mode, when the motors are activated, they turn on gently at the lowest speed making a controlled start easy.
In Atti mode however, when the motors are activated, the propellers spin up at high speed immediately causing the Phantom to flip over and then turn off as it's tilting over.