Thanks for all the feedback and comments folks, I appreciate it.
It was a frantic 2 or 3 minutes whilst these guys ran through their practice routines along various stretches of the Brisbane River.
I had the 7D Mk II set to manual exposure under the C2 pre-set and once I had a few keepers (quick chimp here and there) at 1/1000 sec, I switched to P mode and used the Main Dial to drive the shutter speed down to 1/125, 1/100….1/30 and grabbed a few shots when the helicopters were in hover mode.
When the helicopters recommenced flying along the skyline, I switched back to C2 and had manual settings already dialled in to “ignore” bright backgrounds or dark buildings which would have fooled the auto-exposure setting.
The IS definitely helped here, I could see the stabilising effect in the viewfinder, even though I had both elbows resting on a top rail, with feet spread to compensate for the bumping of the crowds left and right of me!