After a bit of searching I found FExplorer, a free file viewer for the phone. It looked good and had a few good reviews so I downloaded it and tried to install it. Wham! Denied! The phone has the defaults of not allowing software to be installed unless it’s digitally signed. This makes a degree of sense but is quite annoying, so it can be disabled by visiting Tools / Manager then using Options / Settings. Also seemingly of applicability was Tools / Settings / Security / Certificate Management, again under the Options / Settings menu.

Anyway, it’s now installed and works very well.

C also pointed me at a link of interest, which seems to have a lot of information.