When I buy some Wyches I'll give that a go. Thanks, TBE. Another way I have heard is bulding up white in many thin layers on top of a colour like Bleached Bone or whatever the current equivalent colour is.
Adhemar, how did you paint your robes?
Edit: I have finished a test model in white now. I was going to put a picture up, but I think that I may be able to get a good picture tomorrow when there is actual sunlight. Anyway, I'm pleased with the result. I started off with a model base coated Chaos Black, because it was the only spare model that I had. Then I put a coat of thin Codex Grey. After leaving it to dry I did five coats of Skull White. Each one was the consistancy of cream, so some of the grey was still coming through. At this point I did a sharp highlight of Codex grey on everything that was possible to highlight. Then, because it looked too bright I did another coat of thin Skull White before washing it with a very thin, yet dark, wash of codex grey with the tiniest drop of Black Ink. This settled in the recesses and then on the centre or armour plates and on raised areas I placed small amounts of pure Skull White.
It took a while, until I used a hair drier to speed up the process.