Boil the whole oranges in 2 litres water until very soft – it will take about 2 hrs. Pour the liquid into a preserving pan or a large saucepan.
Cut the oranges into wedges, flick out the pips with the point of a knife then gently squeeze the wedges over the pan as they’ll be very juicy. Thinly slice them, then add to the pan with jam sugar. Dissolve over the heat then boil for 7-10 mins, stirring occasionally, until a little of the mixture dropped onto a chilled plate sets and wrinkles when you push your finger through. Leave to settle for 15 mins before potting into sterilised jars.