Turns out the problem was the touchpad driver on my ASUS UX31a laptop, which I was using for right clicking. Updating ASUS Smart Gesture from 1.0.35 to 4.0.13 (...

Just found some more 3rd party context menus in ShellExView (coloured pink) at the bottom the listing - disabling these seems to allow right-click to show context menu for all the problem programs

In some applications (Office 2010, Adobe Reader DC) right-clicking on the screen shows a context menu but then usually hides it again. Holding the right button down or double-clicking sometimes makes...

Followed suggestion of Bree and created a USB installer which does offer the option to use external storage when installing upgrade from there .... BUT ... after upgrade (and creation of Windows.old)...

When I try and run the Win 10 Anniversary upgrade (Windows10Upgrade28084.exe) I am informed that I do not have the 20 GBytes of free space on the C: drive required to proceed and the option to use...