Yeah, what I mean is, I first downloaded the configuration, saw that the setting was already 0, and then just uploaded it again. I didn't actually change anything in between. Anyway, seems to have now got it set correctly. Thanks for the help!
The multiple interfaces problem is odd, since the two interfaces are NOT both connected at the same time. eth0 was just set to DHCP and not connected, but the wlan0 interface only works properly when eth0 is plugged in. If eth0 is unplugged, the wlan interface works for a while but then eventually stops working properly.
This seems to be a Raspberry Pi issue people are reporting elsewhere, though not easy to debug as the unipi display port isn't accessible to connect, so it's hard to see exactly what's going wrong. (Maybe I will take the SD card and boot it in a regular Raspberry PI with a display connected, so I can better understand what is going on.)
The issue would seem to be a combination of the default ARP behaviour in Linux which responds with the wrong MAC address, DHCP, and that when there is no cable connected, in some situations Raspberry Pi still thinks it's up and tries to use eth0 anyway. (/etc/interfaces "auto eth0" vs. "allow-hotplug eth0")
So far, I have both interfaces set to dhcp, and I did have "auto eth0" in /etc/network/interfaces but looks like this was causing some of the problems, so I set it back to "allow-hotplug wlan0 eth0". and then set:
Now, when eth0 cable is removed, the interface still shows as "UP" but it deletes the routes and remove the IP config from the interface, and then because it responds to ARP with the correct MAC address for wlan0, it doesn't break.
Also set "wireless-power off" under the wlan0 interface config, but not sure if that made any difference. It would seem that some wireless APs are able to send a magic APSD request which causes the interface to go into power save mode. I don't think my AP does this.
I'll see how I get on. Worst case, I will have to just add commands to toggle eth0 down when wlan0 comes up, and toggle eth0 up again when wlan0 goes down, called from up / down hooks in /etc/network/interfaces.