Peel the oranges. Cut the flesh into 2.5cm (1-inch) cubes. Set aside. Finely chop the peel.
In a saucepan, cover the peel with cold water and bring to a boil. Drain and repeat two more times.
In a large saucepan, combine the orange peel, flesh, juice, sugar and cinnamon. Bring to a boil and simmer for 1 hour over medium heat, stirring frequently, until the marmalade is translucent. To check for doneness, try the plate test.
Add the lemon juice and cook for 1 minute.
Allow to cool a little then pour the marmalade into jars.
Peeling the oranges means that you remove the peel and the thin white membrane (pith) that protects the citrus fruit segments. With a knife, simply cut off both ends of the orange. Place the fruit on one of its flat bases, to remove the peel of citrus fruit flesh, slide the blade of a knife from top to bottom, then remove any excess pith. This technique is called "blanching" the skin, an important step to reduce the bitterness of most citrus. Plate test - Place a small plate in the freezer for 5 minutes. To check the marmalade is at setting point place a spoonful on the plate and put it back in the freezer for a minute. Id=f the marmalade doesn't run you are good to go - if it does confute cooking and check every few minutes.