1. Having just returned from abroad, I have solved this one before tuning in, as usual. I agree that it is a superb puzzle, apart from a minor flaw.
2. The single manoeuvre required to solve the puzzle, as given by the correct letters to the across clues, should change 20 squares, but only changes 19, since one square does not change or appears not to. This could easily have been avoided.
3. For anyone who wonders if this theme has been used before, it has. When the Times first took on the Listener puzzles it published them in glorious colour, and among those puzzles was one that used this theme Three manoeuvres were required, rather than one as here, yielding an elegant pattern that was familiar to anyone who had played about with the original.
4. Now that the Scrabble problems are printed in colour, it would be good to return to the colourful layout for the crosswords that was used in the early 1990s. Perhaps setters could compose some puzzles that require to be printed in colour before solution. A 15 x 15 based on Scrabble would be an obvious challenge.