Help - Face Recognition/OpenCV using SSH / VNC

I followed the instructions for PanTilt face-tracking.
I don’t have a monitor or keyboard so am using SSH & (tight) VNC.

If I run the program from SSH I get

pi@raspberrypi:~/PanTiltFacetracker $ sudo ./
Frame rate set to 4.000 fps

(Video:1034): Gtk-WARNING **: cannot open display:

If I run from VNC using LTXterminal on Pi I get

“Client is not authorised to connect to server” &
GtK-Warning**: cannot open display …

There is lot on the internet about using different VNC servers, setting DISPLAY=: … all of which I have tried (Although I am shaky on Linux) … I am sure this must be a simple bit of permission setting but am lost :-(