Break the ginger into large pieces and peel. Cut into thin slices and put in a large pan. Peel the lemon rind into wide strips and add to the pan along with the juice. Cover with 1.5 litres water. Stir in the sugar and cook over a low heat for 3-4 minutes until the sugar has dissolved.
Bring to the boil and boil for approximately 40 minutes or until the ginger is very soft and the liquid has reduced to a thin syrup. If it gets too thick stir in a little boiled water.
Remove the pan from the heat and stir in the citric acid. Strain through a funnel lined with clean muslin into a sterilized bottle.
Stand until cooled then seal tightly and label. Store in the fridge, use within 3 months.