Cold solved less than half the clues, but have the grid mechanics, theme, cipher and decoded message, thanks to an early pdm about grid structure (and a related assumption) and the luck of having solved most of the clues that ended up in the SW corner.
With another assumption made, Qat helped me find the first of the losses. from which the rest of the theme followed very nicely. Now just have to solve the rest of the (tough but fair) clues and complete the grid! Am well on the way to saying "wow" to this one - it's certainly worth persevering with...