I seem to have solved the problem.
In my opinion it was caused by the settings on the ROG Realtek Audio Control settings, as when I toggle the "Front panel popup dialog" off and on this causes the Front Panel Headphone/Speakers selection to become selectable (i.e. up to this point it is greyed out). I then changed the Front Panel setting from Headphone to Speakers (or from Speakers to Headphone) then Windows Sound window (from Control Panel) then shows the Speakers as Default Device and audio does come from the front panel headphone jack.
However, at this point while the headphones worked OK, and when I removed them audio went back to the monitor, when I inserted the headphones again they did not work and I had to go through the procedure above to get them to work.
At this point I looked at Sounds in the Control Panel. If I select Properties for Speakers it listed 3 items, all referring to the back panel.
After much pushing it in and taking it out (the headphone jack that is) and playing with settings I got it to work. The settings I have on the Realtek Audio Control Panel are :-
1. Disable Front Panel popup dialog - set to OFF
2. Enable jack detection when device is plugged in - set to OFF.
The Front Panel connection further down the control panel now show Headphone and Mic In, and can not be adjusted from the control panel.
Properties for Speakers still shows 3 items, but the top one has changed from Rear Panel to Front Panel 3.5mm Jack.
Now when I plug headphones in I don't get a popup asking if I have plugged in headphones or speaker. It just says a device has been plugged in.
The Speakers then appear in Windows 10 Settings Window under Audio section.
The Speaker appears in Sonic Studio 3.
The Speakers become the default device in Sounds from Control Panel.
And, most importantly, the headphones work....
When the headphones are removed the sound output goes back to the monitor.
Strange that the settings in Realtek Audio Control Panel need to be OFF to get it to work - Windows doing the switching maybe ?
Regards,
Nick.