Scrub the oranges and place the whole fruits in a large stainless steel or preserving pan.
Cover with 2.25 liters of water and bring to a boil.
Simmer the covered pan for about an hour until the fruit is soft.
Preheat the oven to 140°C and sterilize the jars and lids by washing and drying them inside.
Place several saucers in the freezer to chill for testing the setting point.
Remove the oranges from the pan and measure 1.7 liters of the cooking liquid back into the pan.
Halve the oranges and scoop the flesh, pith, and pips into a muslin bag.
Secure the bag with string and add it to the pan.
Finely shred the orange peel and add it to the pan.
Add the sugar and stir over low heat until fully dissolved.
Bring the mixture to a rolling boil for 10-15 minutes while skimming off any scum.
Boil for up to 30 minutes until the mixture reaches setting point on a chilled saucer.
Ladle the hot marmalade into sterilized jars leaving a 1cm space at the top.
Seal the jars with lids and allow them to cool and thicken.