VNC error - startxfce4: X server already running on display :1

Saw this error in the logs when trying to start VNC on a machine where VNC was earlier working, but was now displaying a blank screen.

https://superuser.com/questions/1607082/tightvnc-in-ubuntu

advises to 

Change the ~/.vnc/xstartup file to:

#!/bin/sh
unset SESSION_MANAGER
unset DBUS_SESSION_BUS_ADDRESS
startxfce4 &

This worked - the session commands were not present before.