Tea was first grown on the mountain slopes around Darjeeling in the 1840s, since when it has become a multi million-dollar export. In that time the basic techniques and machinery used to pick, ferment, wither, roll, dry, grade and pack the tea have changed little. See the entire process during a visit to a factory on the famous Happy Valley Estate. Top-grade, hand-rolled tea from its 150-year-old bushes sells at eye-watering prices in Harrods.