Failing to disable dynamic workspaces on NixOS LiveCD gnome build

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1

I built a NixOS liveCD (see here).

I want to disable dynamic-workspaces. So I have the following in my configuration.nix :

 services.xserver.desktopManager.gnome = {
    sessionPath = [  pkgs.mutter ];
    # Add Firefox and other tools useful for installation to the launcher
    favoriteAppsOverride = ''
      [org.gnome.shell]
      favorite-apps=[ 'chrome.desktop', 'firefox.desktop', 'org.gnome.Nautilus.desktop' ]
    '';
    enable = true;
    extraGSettingsOverrides = ''
[org.gnome.mutter]
dynamic-workspaces=false
[org.gnome.desktop.wm.preferences]
button-layout='appmenu:minimize,maximize,close'
num-workspaces=1
[org.gnome.shell]
enabled-extensions=['no-overview@fthx']
'';
};

I can see that the num-workspaces preference is working on the live CD, but the dynamic-workspaces setting is still active.

Any guidance on what I might be doing wrong?

Many Thanks
Chris