Add the juice of a small lemon and a knob of butter to the macerated oranges and sugar, then bring to the boil.
Stir, then cook for 15-20 min from boiling. At first, stir occasionally, but towards the middle of the cooking time, stir constantly to prevent the jam from sticking to the bottom. Use a skimmer to crush the fruit.
Cooking time will depend on the amount of juice and the degree of heat, so check the consistency on a plate taken from the freezer. Add a teaspoonful of jam, wait a moment and tilt the plate. The jam should set slightly.
Don't overcook your jam to the consistency of a cold jam. At the end of cooking, it should be glossy but runny and fairly liquid. As it cools, it will firm up.
If you thicken it too much, you'll end up with a very thick jam.
If there is a little foam towards the end of cooking, remove it with a skimmer.
Remove from the heat and immediately fill the jars to within 1 cm of the rim, using a funnel if possible. Screw the lids on tightly and turn them upside down on a towel until cool.
After this time, turn the jars right-side up and store in the cupboard, away from light.
Once opened, store in the refrigerator.