Peel mandarins, removing loose pith.
Chop mandarins roughly (breaking up segments into 4 or 5 pieces), removing any seeds. You can also remove any stray seeds later as they float to the top during boiling.
Place chopped mandarins into saucepan and add the lemon (also peeled, chopped and seeded).
Add water and place over heat. Bring to the boil and continue to boil uncovered for 10 minutes.
Add the sugar and continue to boil until the mixture jellies. This should take about 15-25 minutes, depending on the fruit and temperature. When ready, the mixture will start to thicken a little, turn a pale golden colour and will "grab" just slightly to the bottom of the saucepan when you stir the mixture.
Remember to use jam setting sugar!
Pour hot jam into a large sterilised jar. Seal with the lid and allow to cool to room temperature.
Refrigerate and enjoy for up to one month.