Thanks, I accept that advice.
Nothing is ever straightforward with Windows. I turned the slider switch OFF, but then immediately witnessed a notification that periodic scans had been turned ON!
Did a Restart for good measure.
Went into Windows Security again and found the UI now a bit different Opened "Windows Defender Antivirus Options", found the periodic scan switch again showing as ON, so switched it OFF again (screenshot). No switch state notification this time round.
Edit: If you look at my earlier screenshot (reply #7) it was OFF in the first place. When you slide to off it actually changes to ON. So explains my rant above. So from OFF I have turned it ON then back OFF. So why was Defender doing periodic scans in the first place me thinks. I will see what happens in future days.