Wash and quarter apples, but do not peel or core them. Wash mint thoroughly. Put half the mint and the apples into a preserving pan or large saucepan with the water and lemon juice and cook until a thick pulp. Put into a jelly bag and leave to strain overnight. Measure the liquid and add 1 lb. sugar to every pint of liquid. Chop the remaining mint and add. Put over a gentle heat until the sugar has dissolved, then boil rapidly until setting point is reached, about 10 minutes. Add green food colouring, pour into hot sterilized jars and cover tightly.