Thanks Brian and Michael.

Now working OK.

Suspect that the problem was having more than one instance of Moi running.

However, before thinking of that I added the leading underscore to _FxGraph3D.js and _FxGraph3D.htm and altered my files to match which did not fix the problem.
After that I shut all instances down and then started one only.
Then the files worked.

Bit hard to believe that the underscores were the problem so I suspect that while doing this sort of stuff with more that one instance of Moi running is a no-no.