Here's the recipe we used. It's based on one published in The Age newspaper, many moons ago.
1 kg cumquats
5 cups water
2 tblspn lemon juice
5 cups sugar
1. Wash the fruit and cut them into quarters, removing seeds and placing them in a small bowl. Using containers with lids, soak the seeds overnight in 1 cup of the water, the fruit in what remains.
2. Transfer the mixture to a large saucepan and stir in the lemon juice. Bring to boil, then reduce heat and simmer, covered, for 30 minutes until cumquats are tender.
3. Add sugar, stir over heat without boiling until sugar dissolves. Bring to boil over high heat , then continue to cook, uncovered, without stirring for about 20 minutes or until marmalade jells when tested.
4. To test if the marmalade is ready place a teaspoon of it onto a cold saucer and place in freezer for 2 minutes. Remove from freezer and press your finger gently against the marmalade to see if it wrinkles and has formed a skin. If not, keep cooking the marmalade for another 5 minutes, then retest.
5. When ready, remove from heat and allow to settle for 10 minutes. Scoop out any remaining pips at this point and discard. Pour into hot, sterilised jars and seal.
To sterilise jars: Wash in hot soapy water. Rinse. Place upside down on tray in warm oven (120 C) for 20 minutes. Useful to keep in oven while making the marmalade. Sterilise lids by putting them in boiling water for a couple of minutes.