Welcome to the board, Angus! I`'ve been out of the loop for a few years but I'm starting to get my brain back. I've just gone through a similar motor issue. Here is my suggestion.
REMOVE THE PROPS. Get a battery charged and ready. Connect your bird to Betaflight. Go to the motors tab. Connect your battey to the bird. Select the little Slider button that says you assume the risks. It will turn yellow when you select it. Now go to the Master Slider, select it and while holding your mouse button down, start sliding the button up. Are all four motors turning? If they are not, do the same process for motors 1 through 4 and determine which one('s) are not moving. If they all do run, read this thread:
BF Motor calibration and BLHeli issue...
