Hello Neuvi,
Thanks for your comment, but I needed to download a later Windows 11 Administrative Templates pack (for 23H2) https://www.microsoft.com/en-us/download/details.aspx?id=105667
As soon as I copied them to C:\Windows\SYSVOL\sysvol[domain]\Policies\PolicyDefinitions on a DC, I was able to access them via the GPMC on the DC and then create a GPO to disable Copilot.
Regards,
Geoff