Just seen the way the registry key was formatted:

HKEYUSERS\S-1-5-21-1319429059-2914554950-2330803973-1001_Classes\OperaStable\shell\open\command REG_SZ "C:\Program Files (x86)\Opera\Launcher.exe" --private "%1"

There's an underscore between 1001 and Classes which appears to have been interpreted as italicise the preceding word.

Been working now for a week, so seems OK. Thankyou for you help

It appears that the following works - does this make sense?

HKEY_USERS\S-1-5-21-1319429059-2914554950-2330803973-1001_Classes\OperaStable\shell\open\command
REG_SZ
"C:\Program Files (x86)\Opera\Launcher.exe" --private "%1"

I would like to start the private browser if I double click on an HTML document but can't make it work - it only ever starts in the normal browser. Double clicking on the opera icon on the taskbar using --private on the shortcut to get an empty window works fine.

Version: 46.0.2597.39 (PGO)
System: Windows 8.1 64-bit (WoW64)