At least I know what to do for the next one!
The GoPro has some settings that you can alter but they don't really alter frame to frame. Had a test run yesterday to get an idea of where it would be and so on and the same for the pictures as well but until it actually happens you don't really know what settings you have to use. I was originally going to use a variable ND filter for the still shots but, again, after a few test shots I found using ISO 80, f22 and a shutter speed of 1/8000th (basically maxing out the
settings on the camera). At those settings you get reasonably sharp images too. Some were taken with a 200mm and some with a 300mm.
Should be viewable as 4K (for Andy)!