First of all, sterilise the jars. Put the jars in a large pan and fill with cold water. Bring to the boil and simmer for 15 minutes. Carefully remove from the hot water and air dry.
Zest two oranges and set aside. Quarter each orange, scoop out the flesh, removing as much white pith as possible from the fruit segments. Finely slice the peel of two oranges. Throw all the flesh, excess juice and peel into a bowl with 200g sugar. With a gloved hand, thoroughly mix and squeeze, loosening the fruit fibres. Pour this juicy mixture into a heavy-bottomed saucepan, along with the lemon juice and water.
Bring the mix to the boil and skim off the white scum that floats on the top so you have a clear set jam. Let it bubble away gently for 30 minutes, stirring regularly, until the liquid has nearly all evaporated. The peel should be soft.
Add the prosecco and simmer on low heat until it has reduced by half. Put a saucer in the fridge to chill.
Add the remaining 800g sugar and take back up to a gentle boil. To test if it has set dollop a teaspoon of marmalade on the chilled saucer and put back in the fridge for 5 minutes to firm. Run your finger through the cold marmalade – if a clean line appears it is ready, if not bubble away for another 5 to 10 minutes and test again. A loose set marmalade is more desirable – cooked for too long it becomes orange toffee!
Fill the sterilised jars with hot marmalade and seal tightly. Allow to cool.