Ian, thank you in turn for replying, I've gone back to it since and had a longer look at it as it is a 'powerful' portrait, by which I mean one which if it was enlarged to say 20" x 16" and wall mounted in a normal sized living room it would become the centre of attraction.
I don't know how good your processing skills are but I have also seen a few more minor points, which I think would improve it further. The fact that I didn't notice them on the first viewing also shows how well you have captured her 'look'.
They are :-
Removing the small white area in the right shoulder of her top.
Removing the strand of hair on her cheek below her right ear, the odd strands between her right eye and her ear and the strand of hair which has caught the light at the top of her head roughly above her left eye.