I have been making some progress with my head model. I think that some of the things that have been bothering me about it have been solved by reorganising the geometry and moving away from copying the drawing exactly. This has definitely been the case for the nose and the back of the head going into the neck. I've shown some of the progress and the changes I've made below.
At this point I decided that the neck/jaw wasn't working and I needed to change some things around.
I made some changes to the geometry and moved the neck into a different position. I'm much happier with this. I've also done some general cleaning up and sculpting around the mouth and brow.
The next steps are:
Build the inside of the nostrils
Build the throat
Create detailed eyes