John appeared on 'Worst jobs in History' with Tony Robinson twice. Once to demonstrate the natural method of dying cloth with Imperial Purple; the second time to demonstrate how medieval dyers used woad to make indigo dye. Both videos show that John's methods not only work, but how hands-on his research was. It's all a good watch. If you're interested in carrying out any of these methods yourself. The Imperial Purple method is featured in book 7 in the Historic Dyes Series, and the woad method is in book 1 of the same series.
Imperial Purple clip.
Woad clip, part I: NB. This clip starts at 5 minutes in.