Ok, I've got a little further. Before the nodes were connected to "Event Begin Play". It seems I need to add in a small delay, otherwise the nodes don't seem to work, probably because the XRSystem API isn't correctly set yet? I simply have it as a component in my blueprint - is there another way? It seems a bit annoying to work with something like a delay to get the nodes to work.
But then still, I've put in a Time of 5 Minutes for both Sleep and Screen Off Delay, I get the Log Message "Sleep Delay 0" (successfull), but the screen and device still go to sleep after about 40-50 seconds.