Thanks Annie,
This is the sort of picture which can drive a person slowly insane. I've tweaked it again and again and I am still seeing a slightly different image to the one in my mind's eye.
There's something not quite right about the chin and mouth, but I can't quite put my finger on it. If I tweak one bit, it throws something else out of whack, and I'm back to Square One!

If I can ever get it to "feel" right, then I can add some of the detail, but until then its just gonna remain all hazy and undefined