Guys, what I seem to be getting here is that everyone would recommend sticking with the traditional framed, ledged, braced and battened configuration. Fair enough, and I would have gone down this route from the start but for wanting it double boarded which I figured would make the mid-rail and bracing too thin (at 12mm thick). If you think one 12mm thick diagonal brace and mid-rail will give me a door that is sufficiently robust to avoid movement issues then great.
To be clear, you're all recommend doing it this way then: