I have now managed the syncopated diagonals.
The diagonals slope upward from the left, as knitted.
Essentially, this involved reversing the brioche stitch and bringing forward the back colour while sending to the back the front colour.
Now, it isn't as easy as it sounds and the instructions in the book only gives instructions for five rows (only three decreases). Eventually, I did this by looking out for the already turned stitches (these are difficult to spot when initially turned, but the columns of turned stitches decrease by two each time when counting across, so counting the columns helps here).
The dark colour stitch (the first colour yarn to be knitted through) is turned. During the light colour 'turn', the stitch immediately to the right of the turned dark one is turned. On the wrong side of the work the stitches were worked as they appeared.
I also got bored with pink and so changed to turquoise.