I applied the eye rigging I had just learned to the file I have been using for all of my facial rigging so that I can start animating. The process went well I got everything right until...
When I moved the locator the eyes started spinning out the wrong way and when I tried to rectify it they ended up like this on different levels:
I believe the cause of the problem is that the axis for the eyes were not in the centre of the eyes themselves but actually at the bottom of the head rig, so when they turned for the locator it was affected by the placement of their rotation.
I am going to start again and see if moving these to the centre of the eyeballs makes a difference.