I can’t find any option to turn off the auto lock screen. I’ve installed the screen saver and unticked “lock the screen” and in power management I have “put computer and display to sleep” both never. But the display goes blank and needs my password to reappear.
Well I’ve sort of fixed it. Uninstalled gnome screen saver and installed xscreensaver instead.
But even though gnome screen saver is no longer there, I’ve now got two screensavers! But the display is no longer locking, so I’m not too worried.
Woops, don’t know why I didn’t see it before. The “Mate-screensaver” is an integral part of the distro. I can’t uninstall that without uninstalling the whole desktop!
Looks like running the gnome-screensaver as well as the mate-screen saver caused the problem.