Place the oranges and lemons in a large, deep saucepan, then cover with 2 litres of water and bring to the boil. Simmer for 1½ hours until tender.
Remove the fruit from the saucepan and allow to cool slightly, then cut into quarters and scoop out the pulp.
Place it in a colander over the saucepan of cooking water, press down with the back of a ladle to squeeze out all the juice, then add the pulp and discard the pips.
Discard the lemon peel, then finely slice the orange peel and add to the pan with the sugar.
Bring to the boil, skimming off any white residue. Continue to boil rapidly (so you have a bubbly foam on top of the mixture) for 40 minutes until it starts to set.
Leave to cool for 20 minutes, then stir to distribute the peel.
Wash empty jars and place into an oven on a medium heat for 5 minutes to sterilise, then ladle in the marmalade and seal with a lid.
Cook's tip: To find out if your marmalade has reached setting point, pop a small plate in the fridge while it is boiling. After the 40 minutes, drop a small spoonfull of the marmalade onto the cold saucer. Let cool in the fridge for a few minutes, then gently push the marmalade with your finger. If it forms a crinkly skin, the marmalade has set, if not, boil for another few minutes and check again.