By trial and error I think I figured out how to resume GUI mode if in Headless Mode:
1. Power down the Id
2. Insert HDMI and keyboard & mouse cables
3. Boot the Id
4. Observe standard boot images until a logo (a logo related to the GUI mode I think) flashes on & off.
5. Then type the number "1" and then "enter". (Though the monitor is not active because of headless mode, I believe the text mode menu is active and so is the keyboard input.
Consequently you can type any of the text menu numbers i.e., "1" for GUI and cause a reboot into GUI mode which replaces the headless mode.)
6. After a delay the command lines will start scrolling and the GUI will soon appear.
I am amazed that everyone one who read my initial post did not know how to back out of the headless mode or was unwilling to comment.