Place the lentils and 900ml/1¾ pints of the water into a pan, stir well and bring to the boil. Skim off any froth that forms on the surface of the water with a spoon. Cover the pan with a lid and reduce the heat to a simmer. Simmer, stirring regularly, for 35-40 minutes, or until the lentils are just tender, adding more water as necessary.