1. Line a sieve with muslin (or a small piece of thin, clean cloth) and set it over a bowl. Halve the oranges and lemons and squeeze their juice into the sieve, so the cloth catches the pips and pith. Set the bowl aside to let the juice drip through the cloth. Discard the juiced lemon peels (or keep it for use in other dishes).
2. While the citrus juice is filtering into the bowl, shred the orange peels. Cut the orange halves in half again so you can flatten them on your chopping board. Don't worry about any pulp or pith still attached, it'll add flavour and texture to the marmalade. Slice the peel as thick or thin as you prefer, remembering that it will plump up as it cooks, so slice it a bit thinner than you'd like it to end up. If you come across any more pips, add them to the sieve over the bowl.
3. Gather up the cloth full of pips and tie it into a bundle to keep the pips inside. They contain pectin, which will help the marmalade set.
4. Pop the bundle of pips in a large sturdy pot along with the shredded peel, citrus juice and 4 pints of water. Cover and set aside to soak at room temperature for 24 hrs.
5. After 24 hrs, set the pot on a medium-high heat to come to a gentle boil. Cook for about 1 hr 30 mins, till the orange peel has become soft. Carefully remove the bag of pips from the pot and squeeze it gently to wring out any remaining juice and pectin. Take the pot off the heat.
6. Add 500g caster sugar for every pint of liquid and peel in the pot. You should need around 1.5-2kg. Stir till the sugar has completely dissolved, then pop the pan back on the heat and bring it to the boil. Boil rapidly for 30 mins-2hrs, till the marmalade's setting point is reached. The amount of time depends on the pot you use and the amount of pectin in the pips, so keep an eye on the marmalade. When it looks thick and sticky, do the set test.
7. You'll know your marmalade is done when you spoon a little onto a cold plate and let it cool. The surface should be set, and it'll go wrinkly if you push it gently with your finger.
8. Pour the marmalade into warm sterilized jars (see our tip) and seal right away. Store and enjoy till next year’s Seville oranges are in season.