EE UK (EVR) 5.1.1 installation of non-play store apps (unknown sources)

Since the EE Galaxy S6 firmware upgrade to 5.1.1 I have not been able to install (or update) apps from "unknown sources".
I don't use the Play store at all - I use F-Droid - so this means I cannot use F-Droid as a package management tool! :(

The "allow unknown sources" setting still exists and is enabled, but all installs or updates just hang after confirming the requested permissions...

Has anyone else noticed this problem, or might know of a way to rectify this situation?

Thanks