"Gert. You might try making sure that on step 4 there is a half a turn."
There should be a half turn between steps 3 and 5, but there is no half turn of pivot on step four. Instead, the body turns into the step, the foot pivots slightly, and then body turns a lot more out of the step. This will substantially help with hitting step 5 with both the spin and the balance you will need to finish the figure. Think of it less like a natural pivot turn, and more like steps 1-5 of a naturl, only spin when you get to the ball of the foot on 5.
"Dont twist the spine."
Twisting the body around the spine is rather essential to the natural CBM that makes the figure work.
"What I used to think was an Overturned Spin Turn wasn't. It was just a normal Spin Turn to finish backing diagnal to the center with the LOD. An under turned Spin Turn would finish backing diagnal to the center against the LOD. A Overturned Spin Turn will finish backing the LOD ready for a Turning Lock or whatever."
That is the correct alignment, but there really isn't much difference in the various actions - the overturned one is just a little bit more so. I suspect that someone having problems with the overturned spin turn can't dance one to the less turned alignment cleanly either, and it might help to practice those for precision firts.
"You will find the make or break step for both man and lady is step four. I do find that unless they have been taught the ladies are inclined to hang back a bit on that step, which means the man has to use his sturdy arms to get her around."
Ah, that's a clear problem. The lady has to be willing to project her body past her standing foot as she lowers from step three with her feet closed. It is extremely hard to do this - much harder than what the man does via a prep step into step 1 of the natural.
The man should not be using his arms to torque the lady, but he should turn his body even as he is placing step four, as this early turn of the body will allow the lady to project her whole left side as she takes step 4 - in that position she can dance a substantially more powerful action. Again, it is not a natural pivot turn - it is a spin turn, which a slight hint of the ordinary 45 of natural action rather than just a pure pivot.