@frog OK, according to the Jessie Lite instructions( https://github.com/MichMich/MagicMirror/wiki/Jessie-Lite-Installation-Guide
, I installed:
sudo apt-get install lxde-core
sudo apt-get install lightdm
so - it is LXDE UI and Lightdm being used. Which should mean that the alterations to /etc/xdg/lxsession/LXDE/autostart and sudo nano /etc/lightdm/lightdm.conf should have worked. But they didn’t.
Anyhow, I fixed it the old fashioned way by uninstalling unclutter, setting the raspberry to boot requiring user login then attaching a USB keyboard/mouse to the raspberry and setting the screensaver off in preferences. Hopefully it’ll stick!