Put the rhubarb in a bowl, pour over 4 cups/1 litre boiling water, cover, and leave at room temperature overnight. Next day, strain the liquid into a saucepan, discarding the rhubarb. Add the sugar and lemon juice, and bring to a boil 5 minutes. Cool. Taste and add more lemon juice if necessary. Strain into a bottle, and cork. Store refrigerated, to be served ice-cold.