Back from work and render still running after 19 hours. It's only 750x750. I think it is the AO being calculated on the reflection of the transmapped hair, as raven suggested.

I'm using AO on my IBL, not MBAO. Perhaps I'll try a render without IBL AO. If that completes, I might try it with MBAO on the figure and wall, and not on the hair.

But by the time I get an acceptable render of this image, I'll probably have gone off it anyway....

"If I were a shadow, I know I wouldn't like to be half of what I should be."
Mr Otsuka, the old black tomcat in Kafka on the Shore (Haruki Murakami)