Thanks to the Thinkwiki xrandr tutorial I can now dual-monitor my laptop.
I had to rebuild xorg.conf, as the gnome screen resolution tool had messed it up. Then it was a simple matter of following the instructions.
One important gotcha – the default refresh rate was too much for my old LCD monitor to handle, so I had to turn it on using:
xrandr --output VGA --mode 1280x1024 --right-of LVDS --rate 60
Which seems to have worked nicely.