3 lemons
1 kg sugar
50 g citric acid
1.5 litres water
Place the sugar, acid, and the lemon juice, pith, pips and thinly removed peel in a large bowl. Bring the water to the boil (in a kettle is easiest), and pour over the other ingredients. Stir until dissolved. When cool enough, place in bottles and keep in the refrigerator until used.
Dilute to taste. This amount fits very comfortably into a 4-pint plastic milk bottle from the supermarket!
I find it keeps a great deal better if you strain it before bottling, as it is the lemon peel that is apt to go mouldy first! But it doesn't last much more than 3 weeks, so give it a 3-week "use-by" date.