Changing the screen resolution

System:
Thinkpad 400 Laptop
OS: Trisquel 8, Based on Ubuntu 16.04 LTS
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Hi

The screen on my Thinkpad 400 laptop is broken, so I have hooked it up to an external monitor - Philips 226E9Q, via VGA.

When I did this I noticed the screen resolution settings were incorrect, so I could not see the whole display.

I have managed to correct this by using the following commands:
# check aavailable outputs and modes
$ xrandr -q

''''
lists outputs and modes one of which is VGA1 / 1920x1080
....
# set required mode for VGA
$ xrandr --output VGA1 --mode "1920x1080"

Doing this allows me to see the whole screen, however this needs to be done separately for every user - the setting is not global.

Having set the resolution I can see the menus and can now try to use the GUI to set the resolution.
I go to ControlCentre->Display where in theory I can set the resolution, but the list in Resolution field does not give me the option of 1920x1080 (even though xrandr lists that mode).

Does anyone know how I can add the required mode to the list presented in the Display dialog ?
(dialog has a 'Apply system wide' button which I'm hoping will set it globally when I can select the mode)

Failing that (or actually as well) does anyone know how I can set the resolution globally from the command line ?

Also does anyone know where the resolution settings are stored ? Presumably there is a file or something I can edit to set them manually.