Calling all aspiring spooks. Robert Hannigan, director of Britain’s security and intelligence organisation GCHQ, has included a rather tantalising puzzle with his Christmas card this year. He hopes that it will exercise your grey cells over the holiday period. If you can solve the puzzle, along with the others that it will lead to, you can email the solution to GCHQ (the Government communications headquarters) before January 31. A winner will be drawn from all the correct answers – and doubtless be named to much fanfare. So what do you need to do to be in with a chance?