At the risk of trying people's patience, I've put up some more video here which I'd forgotten we'd taken, and I'd be interested in any feedback.
Firstly, here's one done on a slightly gentler slope than the others I posted - still a red, but not a steep pitch, the only difficult being the wet, rucked up snow. This was one of the video session which made me rattled, because a small group of people stopped just above where my partner was waiting to film me and didn't move for ages (I thought they'd taken up residence there), then just as they moved away, a ski class came down the slope. I waited till they had disappeared over the horizon to the left and then set off, intending to ski past my camera-wielding partner, but had to hurriedly abort because the class had come back across the slope (I really thought they'd gone down).
Secondly, here's the top of a bump run under a lift. This shows the rather benign top section - further down, just beyond where I stop, the bumps get much higher and tougher.
Incidentally, I've been looking at all these videos, frame by frame, using the open source video editor Avidemux, and it's something I strongly recommend people do with their own video, as it's amazing what a frame by frame analysis reveals. It's really given me a lot more information about what I'm actually doing, which is often difficult to make out in normal video viewing. It has confirmed that at least some of the time I am actually releasing by flexing and tipping, albeit in a probably incorrect way. The section from 15.5 to 16.5 seconds in the first video shows a typical turn that I'm making in these conditions, where I am strongly pulling the stance leg out of the snow such that I actually take off momentarily, tipping to the little toe edge with the free foot. But the weird thing, which I can see on other turns as well, is that the tip of my free foot ski is actually further off the ground than the tail, something which Max_501 spotted in one of the other videos. Presumably this reveals something about my fore/aft balance, and I imagine the cure is to concentrate even more on pulling my feet back under my hips.