So great having all this encouragement.
The beet juice fabric kind of worked. When I got home it was quite a nice brown and the resist was breaking down. So, a quick rinse and leave to dry overnight. Well, this morning I have brown fabric. It leached into the design so there are no witch's feet at all. Still, I now have a nice colour to start another batch with. I started with beet juice and now I'm making lemonade.