Fn+F8 does not switch displays on a Dell E6410
Finally I gave up and decided to adopt the simpler solution of using disper.
apt-add-repository ppa:disper-dev/ppa
apt-get update
apt-get install disper
and then
disper -c to switch to external monitor and disper -s to return to main screen. I also configured a shortcut in xfce which I now use to map p to disper -c
Better solution for me seems to be to switch external display on/off using nvidia-settings, where it is possible to detect resolution of external display and switch the TwinView feature on and off (as well as other helpful configuration options).