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.
Because of the non-turning/turning, I had to rip back and start again - luckily before I'd got too far.
Then there was the issue of weaving in ends. I initially started doing that after every few rounds or so. Then I'd discover a mistake in the pattern and have to rip back, which is actually quite difficult with ends weaved in. I decided to leave that until the end.
Today, I have spent mostly weaving in ends.
Of course, the mistakes didn't end there; I joined together the wrong handles at first <sigh>
After all that, it might be a while before it gets lined, but it's finished (and so am I for the rest of the day).