Short post as a note to myself.
My tutor looked at my animation today, particularly the swivelling problem.
Apparently this is caused by the rigging of the tail being made with a 3D chain rather than a 2D chain.
Note to self, 3D chains are harder to control. 2D chains would probably have been better!!
To fix the problem in the animation (without re-rigging the model and starting again) I will have to key ALL frames inbetween the keyframes.