@AndrewR THANK YOU!
that was the problem. Or more accurately most of it.
It seems things are very fussy about the order of doing things (at least on both macs I have access to).
0. Install all software, wait for reboots.
1. Make sure webcam 2 is *not* running.
2. Start up a *qualified* app, such as the most recent zoom or chrome version of the likes of MS Teams (I'm starting to prefer Chrome MS teams to native...!)
3. turn the camera on.
This works both with usb-c to usb-c and usb-c to usb-a via an anker hub. performance is pretty good, though I know hub is USB-3, so lots of bandwidth (I'd guess if it negotiated down, you may run into issues of quality as a result, so USB-C is probably safer where possible).
Once you've got it working it is *much* more tolerant of the order of 1, 2 and 3, *provided you don't reboot*. After a reboot I've found that doing in a different order failed (okay only once as I've only done a few reboots. For me, having the camera turned on before starting the app seemed to cause problems (webcam 2 perhaps only listens for a "new camera" message before doing something, and can only do that once the app has started, but once its done the initialisation it doesn't need to repeat things even when you quit the app.)