Combine all the ingredients in a large saucepan and simmer over a medium heat for 5 minutes, then reduce the heat to low and continue simmering for 10 minutes. Remove from the heat and leave to cool for 1 hour.
Strain the juice into a jug. Chill for 1 hour, or serve over crushed ice.
Variation: to serve the drink hot, strain the juice as in Step 2, then reheat gently and serve it with a cinnamon stick in each glass to stir.