..\launcher.exe --private WORKS!! YABADABADOO!!
..\launcher.exe -newprivatetab doesn't work. Don't (was probably ok earlier versions, is not now)
I'm on V25.0.1614.50. Many thanks to all posters this thread. Happy chappy!
FYI I'm W7 and I've restored the original Quick Launch bar, where I have Opera as a shortcut. It's easy to right click the properties of the shortcut and edit. NB make sure all versions of Opera are stopped and not running (maybe confirm using Task Manager) before trying the new shortcut. HTH