Have discovered that it's to do with the Fn key. As a local user pressing Fn + / produces +. So, as a regular user it appears the Fn key is automatically being pressed. I assumed therefore that pressing Fn as a regular user would be the reverse and then type the correct character. Not the case! Appears that the Fn key is effectively being held down. Now struggling to find a way to disable it completely.
Update: Found a BIOS setting to change the Fn key function (or force it to be on or "default" off). Again, if the standard user login acts like Fn is pressed, I was hoping this, confusingly, might reverse it. No! It works as far as the F keys are concerned (forcing the Fn function without pressing Fn) but doesn't fix the / key fault.
So what we have here is a standard user that regardless of the Fn settings applied insists on being configured with the Fn key pressed.