I've just given up hope of getting gnome-screensaver to work for me, so now I've having a go with xscreensaver instead. I've followed procedure recommended as in http://www.liberiangeek.net/2011/10/...neiric-ocelot/
... having done ...
1. Removed the gnome-screensaver one
2. Done sudo apt-get install xscreensaver xscreensaver-gl-extra xscreensaver-data-extra
3. Added xscreensaver -nosplash to my Startup Applications (and re-booted)
4. Found xscreensaver-demo screen (not in Dash as recommended, since I'm on Classic Gnome, but it's just the same screen as in the Liberian Geek howto article)
5. Set all my options to 1 minute
6. (including successfully previewing my chosen screensaver)
7. Waited a minute -- and waited...
8. Also tried invoking the screen lock, both from the login/shutdown options at the top right of my screen, and by Ctrl-Alt-L. That doesn't work either. The only thing that *does* work is if I open Terminal and invoke xscreensaver -lock.
9. Started to wonder if I'm being really stupid!
Grateful in advance!