Grate the zest of the lemons and squeeze the juice out. Place in a bowl. Now cut up the lemons and add those to the bowl. Place the sugar in a very large bowl or something that will hold that and the water. Pour the boiling water over the top and leave to cool.
Now add the tartaric acid and the lemon juice, zest and chopped skins. Leave in a cool place for 3 days – stirring occasionally whenever you pass it. Strain through some muslin and bottle. It will keep for about 5 weeks in the fridge.