Update -

Interesting that you say your thumb moves all over the place mindwarper. I've come to realise that it's fairly normal. From another thread (on the right hand pinky) I saw this video -

http://www.youtube.com/watch?v=AgCt5fcFOog

and it illustrates that the thumb can be pretty mobile and playing isn't adversely affected.

After me focussing on it my thumb position has 'improved' as I'm now able to see the advantages of good technique but also that practical considerations come into play like what's comfortable for me. Now I can accept that my thumb position might not be 'correct' but at least I've looked at it and improved things.

Time now for me to move on and concentrate on making progress through the lessons....