Thanks Ralph. It's been an almighty struggle with the head.
Did some more work on the face, then on to the helmet:
There really is nowhere to hide at this scale - every lump and bump shows. Things were not helped by the helmet flying off and hitting the floor. The main casualty was the nose-guard which shattered into three pieces. I had to shorten it to cut off the damage, but I think it looks OK.
To paint, I first gave the helmet a coat of flat black, then a layer of matt varnish to protect it. I next dry-brushed on Vallejo Oily Steel followed by Natural Steel, building up the highlights with layers of Oily Steel. Then I consolidated the lines and rivets with thinned black, and finally dotted in the rivets with Old Gold highlighted with Pure Gold. Still a bit of burnishing to do.