xfce4-power-manager problem, display no longer turning off, 15.04 and 15.10

[Chris Green]

My laptop and desktop displays no longer turn off after inactivity. It
was all working as it should until recently, I think it stopped
working a few days ago or maybe a week or so.

Until very recently both systems were running xubuntu 15.04, I have
now upgraded the laptop to 15.10 but the display is still not
blanking. 

I don't think I have changed any relevant settings, everything still
looks right in the Settings Manager.

What's going on, does anyone have any ideas?

-- 
Chris Green