I think its necessary to rename the file but that is not sufficient.
It was only after I had been playing the file with other Media Players that eventually it worked. Maybe not even the same day. I can't remember exactly how but. The first Player I used was QuickTime and the next Media Player I used was Media Player Classic. Then some time later tried again with VLC and it worked. MPC comes with the free K-Lite Codec package.
Success at last (I think)... Maybe you are right just rename, but not just the duplicated .mp4, but also all the extra characters prefixed ie brief renaming, I just now tried using only brief renaming and it worked! I don't know if it will keep working . I actually did the rename in the same dBox that came up to Open File - a twofer so to speak.
Fingers crossed - its alot easier than all the palaver I described above
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