Stop press -- **new bug found which will change this...**
While investigating various failing tests, I found that the original had a bug in its handling of decreasing coordinates, whereby it doesn't reverse the *data* order according to where it has reversed the coord points.
This was the point of the supposedly "redundant instance variable", the bug is that it should have been **used** !

Fix to this branch to follow shortly ...
  -- though the same fix will probably need re-writing to apply to @bjlittle's later https://github.com/cpelley/iris/pull/13