So I went to Woolfest and asked several people how to ply. The spinners were their usual generous selves, and gave me the benefit of their experience. They allowed me to watch them ply, and one spinner even allowed me to have a go on her wheel.
Then I came home, did everything suggested to me, and produced the above sample. Misery and frustration ensued. I rang a friend to have a moan and she simply asked "Have you checked your singles? You may be over spinning them"
It sounds like the Golidlocks story, but with yarn LOL