Thanks stewer. I've been playing around with different settings all day, and [this page](http://graphics.smithmicro.com/go/p6tutorial2) was a big help, even if it is outdated and slightly vague.

Attached are my latest settings, and while they're much faster than before, I think they could still do with some trimming...at least it doesn't take 10 minutes.  :-P
For lighting I'm using the default "Lagoon Desaturated" IBL, plus a skintone HDRI from AerySoul described here (NWS).