Skiing happened again yesterday. I'm starting to really relax and enjoy it. We practiced the 'Snow Plough' again, from higher up the slope. He got us to move into and out of the plough from parallel as we slid. This was relatively easy to do.
Then we used the plough to turn. Essentially the idea is that if you want to turn right you sweep out your left leg. This points the plough to the right, and off you go. Another way to thing about it is to 'push down' on the left leg a bit (without going on one foot). When it was explained to me as 'pushing down' I could not make it happen, but the 'sweeping out' worked reliably for me, and I found that when I tried to 'sweep' the 'push' happened automatically.
The trick is that this works better when you have a bit of speed, when things are slow, its quite difficult as the skis have to be fought.
So, I can now slow down (a snow plough won't stop you on snow, though it stops on a dry slope), and I can turn. On snow, turning allows you to stop - essentially you keep turning and kill speed by going along and up the slope a bit, before ending up parallel to the slope.