As you can see in this photo of my kit, the left rack arm is under some pressure to not twist.
Anyway, for those with that issue here is my fix.
First I found a long 8mm bolt and a matching drill bit.
I then drilled an 8mm hole right through the rack fitting and the pipe within.
And I stuck a bolt in it.
That pipe is not ever going to twist.
Job done.
Almost.
I cut the bolt shorter and filed it smooth on the end.
For now I have just stuck a bit of gaffa tape on the end to stop it shaking loose. But I will try to find some sort of simple clamp to hold it. It only needs to hold the bolt to stop it slipping out.
These rack pipes will never twist.