Scrub the oranges and place the whole fruits in a large stainless steel pan, or preserving pan.
Cover with 2.25 litres/4 pints water, then bring to the boil. Reduce the heat and simmer, covered, for about an hour until the fruit is soft.
Preheat the oven to 140C/275F/Gas 1. Wash the jars well in warm soapy water then rinse thoroughly under running water. Leave the jars and lids to dry, upside down, in the oven. Place a few saucers in the freezer to chill (these will be used to test if the cooked marmalade has reached setting point).
Remove the oranges from the pan with a slotted spoon and set aside to cool. Carefully measure out 1.7 litres/3 pints of the cooking liquid, discarding any extra or topping up with water as necessary. Return the liquid to the pan.
When the oranges are cool enough to handle, cut them in half and scoop out the flesh, pith and pips into a bowl. Pour the orange pulp into a muslin bag and secure with kitchen string. Add to the pan.
Chop the peel into shreds as fine as you like and add to the pan. Set the pan over a low heat and add the sugar. Stir until the sugar is dissolved.
Bring the marmalade to a rolling boil for 10-15 minutes. Skim off any orange scum that rises to the surface.
Test for setting point by dropping a little of the mixture onto a chilled saucer, leave for a moment, then push your finger into the marmalade. If it wrinkles it is ready. Alternatively, dip a spoon into the marmalade, allow the mixture to cool a little, then slowly pour it back into the pan. If it is at the setting point, the drops will run together to form a hanging flake (this is known as the flake test). It can take up to 30 minutes to reach setting point, so keep testing.
When the marmalade is ready, remove the pan from the heat. Carefully ladle into the hot sterilised jars (a sterilised jam funnel makes this much easier) leaving approximately 1cm/½in space at the top of the jar. Twist the lids on the hot jars to seal. The marmalade will continue to thicken up as it cools.