It was the brushwork on this one that was the challenge. That and not really understanding what I was looking at, especially at the nose end (the photo was quite poor and badly lit). I had a couple of wipers and was on the verge of giving up.
This result came from a decision to paint exactly what I could see - although pushing the value range - even though I wasn't too clear what it was - and lo and behold a nose materialised. Magic. I used one of my cheap, unbranded nylon brushes. I wonder why there is so much advice and wisdom out there to the effect that you must use " the best" (ie most expensive) brushes?