Thanks for the comments all. (sorry to hijack the thread....)
The robe was done by a sort of reverse layering method.
1. I undercoat with white primer.
2. Wash all the folds of the robe (even the shallow ones) with really thinned light grey (I used GW Fortress Grey) - less is more; you can always come back and add more grey later if needed!
3. Especially deep folds I washed with thinned GW Codex Grey right in the depths of them
4. Re apply pure white to the raised parts of the robe with a layering technique to add depth leaving a touch of light grey in the light folds, and the darker greys blended into the heavy folds
Try it! It is all a bit of trial of error. The worst that could possibly happen is to strip it and start it again (one reason I am loath to buy plastic figures...) but more often than not I am pleased with how my experiments turn out.