Cool. I appreciate the suggestions.
I’ve just checked the ESCs for MOT_SPIN_MIN and it works out to 8%. As the default setting is 15%, I only assume that when there is a reverse wind blowing (i.e. in freefall) you need much higher than 15% to counter the wind acting against the props.
As I cannot check what this value should be unless I have a wind tunnel or drive with my drone held out the window, I will use Air Mode in future to guarantee stabilistion. If I understand it, I just need to map a switch on the Tx to ARM instead of using the traditional low-throttle-right-rudder method.