Now that's an interesting idea, but before going down that route - my update...
I recalibrated the forward and downward vision sensors, compass and IMUs this afternoon. Whilst looking at the photo of the front left vision camera, I noticed that the top cover of the drone was slightly separated from the body, so pressed down on it and it moved a fraction and "clicked" into place. I'd watched a Mavic tear-down video yesterday afternoon and the guy in the video commented something about the pressure from the top covers holding the connectors for the cameras (and antennae?) in place. Interesting. So, I waited for the rain to ease off this afternoon at home and took the Mavic out for a test flight. Take off to hover and OA radar sensors reporting obstacles in front about 4m away - the fence. Took the Mavic up to 3m so it was looking over the fence and no obstructions for 10m - radar all green. Full stick forward and my demented dragonfly tipped over and took off at what looked like normal speed. Hallelujah! I couldn't take it for a full run - not enough space, but I'll put it through its paces tomorrow in the work car park and test it, including RTH. Keeping fingers crossed that she might be good...