I got to the root of the problem last night, partly an issue with the MK frame the Hoverfly board is sitting on, partly an issue with the code.
Here's what's happening, after the board is powered up but before its armed the camera mount will respond to tilt commands from the TX as well as compensation commands from the board. Once the board is armed there is a difference in the state of the camera mount, it still responds to both the TX and commands from the board, but the board appears now to have a dampening effect on the output of what it sends to the camera mount when in an armed state.
Briefly, with the quad on the bench and powered up but not armed, if I tap on any part of it and induce a small amount of vibration the camera mount moves in reponse, something it will not do when armed unless the frame makes a much more substantial movement. Once the mount swings a bit when unarmed it induces yet more motion which then causes the highly flexable MK landing gear to sway a bit, introducing even more motion which the board picks up and sends back to the mount as a highly amplified command to compensate for the sensed motion. From that point it just feeds on the now steady swinging motion of the camera mount to keep the cycle going.
Putting a finger on the frame to dampen the vibrations causes the camera mount motion to stop, tapping it again will cause it to start up again. Likewise arming the board also stops the movement as the board now appears to filter and ignore the low level vibrations, not sending them on as commands to compensate for the percieved motion. No amount of tapping or banging on the frame will cause the uncontrolled swinging to start while armed. Disarm the board and I can easily start the cycle again with just a tap of the finger.
Prior to the upgrade I had been flying with an older version of firmware because I didn't like the way the camera mount was working with the latest versions. Something that was done to the code in subsequent releases increased the sensitivity of the boards response to movement to get the mount to respond and recenter quicker, the side effect is what you see in the video if the conditions are right. A classic example of how fixing something in software can easily cause unintended problems elsewhere in the system.
Ken