Preheat the slow cooker if necessary; see the manufacturer's instructions. Put the whole oranges into the slow cooker pot, cover with the boiling water and put an upturned saucer on top of the oranges to stop them from floating.
Cover with the lid and cook on low for 8–10 hours or until the oranges are tender. Lift the pot out of the housing using oven gloves and leave to cool overnight. The next day, lift the oranges out of the slow cooker pot, draining well. Cut into quarters, scoop out and discard the pips, then thinly slice the oranges.
Put the sliced oranges and the liquid from the slow cooker pot into a preserving pan or large saucepan, add the sugar and heat gently, stirring occasionally until the sugar has completely dissolved. Increase the heat, and boil for 20–30 minutes or until setting point is reached (see
Warm 6 clean jars in a low oven for 5 minutes. Ladle the hot marmalade into the warm jars, cover the surface with waxed discs, add cellophane jam pot covers and secure with elastic bands or screw the jar lids in place. Label and store in a cool place until required.