Changing Monitor that gdm uses

[Alan Chandler <alan@chandlerfamily.org.uk>]

My desktop Debian Jessie PC  just got a new monitor to replace an old 
one that failed. Because the new one is bigger than my previous main 
monitor, I have swapped them round so that my bigger 27inch monitor is 
the primary one and the original 24inch one has become my secondary.

I have altered my /etc/X11/xorg.conf so that the new one is the Primary one.

Section "Device"
    Identifier "H67 Integrated Graphics"
    Driver "intel"
    Option "Monitor-HDMI2" "Iiyama 27in"
    Option "Monitor-HDMI1" "Iiyama 24in"
EndSection

Section "Monitor"
    Identifier "Iiyama 27in"
    Option "DPMS"
    Option "Primary"
    Option "Position" "0 0"
    Option "PreferredMode" "1920x1080"
    Option "Position" "1920 0"
EndSection

Section "Monitor"
    Identifier "Iiyama 24in"
    Option "DPMS"
    Option "PreferredMode" "1920x1080"
    Option "Position" "1920 0"
EndSection

Section "Screen"
    Identifier "Default Screen"
    Device "H67 Integrated Graphics"
    Monitor "Iiyama 27in"
    DefaultDepth 24
    Subsection "Display"
           Depth 24
           Modes "1920x1080" "1024x768" "640x480"
    EndSubSection
EndSection


But gdm3 has decided it is going to come up on the secondary monitor.  
Once the desktop starts the order of the displays is probably* also 
wrong, but of course can be fixed in the display settings

* I say that because the main monitor is numbered 2 and the old monitor 
is numbered 1.

Where is the detection of which monitor is which take place, and how do 
I tell it to change the order?

--
Alan Chandler
http://www.chandlerfamily.org.uk