I've been fiddling with my lightbox to get better photos - something closer to the colours I see when looking at the model. Below is the journey I took with my Ork to get to a good place.
The default option with lightsource above lightbox, diffused.
Changed to increase exposure.
Changed the camera's elevation to be a little higher.
Increased the exposure again, without moving the light source.
Increased f-value to f11 but felt it was leaving the background in sharp focus as well now.
Pushed the light source back to be over the camera instead of over the lightbox. Getting close now.
Realised I'd overcomplicated life a little. Moved the light back to almost overhead the miniature again, and downstepped the exposure.
There will be further adjustments over time, but for the moment this feels pretty good - and gives an accurate representation of what the model looks like in real life. Here's how the camera setup looks in reality.