Photograph: Martin Poole
Halve and juice the Seville oranges, then scrape out the pith and pips on to a square of muslin and tie into a bag.
Finely slice the orange rind and add to a preserving pan with the juice, water and the muslin bag, immersed in the liquid and tied to the pan handle.
Simmer gently, uncovered, for 2-2½ hours until the orange rind is soft.
Remove the muslin bag and squeeze as much of the juices into the pan as you can. Discard the bag.
Add the granulated sugar, brown sugar and the juice of the lemon to the pan. Stir to dissolve, then boil for 15 minutes. To check if it's set, chill a teaspoon of marmalade on a saucer for a few minutes - it should wrinkle when pushed. Boil for longer if necessary.
Cool for 10 minutes; pour into hot sterilised jars and seal.