Wash the oranges and put them in a large pan along with the water. Bring it to the boil and then reduce the heat to a simmer; leave for a 3/4 of an hour, or until the fruit is soft (the length of time will depend on the size and ripeness of your oranges).
Whilst the fruit is softening, wash your jars in hot water and then dry, upside down, in the oven (about 100ºC), along with the lids. Put a saucer in the fridge to cool – it’ll be used for determining if the marmalade is set.
Remove the oranges from the pan and set them aside to cool. Keep 1 and a half litres of the cooking liquid, ditch the rest.
Half the oranges and scoop out the innards, putting them into a bag made of muslin. Take the rinds and slice them thinly (or as you like your marmalade). Put the peel into the pan and set it on a low heat. Add in the sugar and stir until it is entirely dissolved.
Add the muslin back of the flesh and the gin. Turn the heat up so that it boils. Boil for 15 minutes, skimming off any scum that comes off the oranges.
To test the setting point, drop a little of the marmalade on to the cold saucer (that you put in the fridge earlier). Leave it to cool for a few moments, then run your finger through it. If it wrinkles around your finger, or up ahead of it, then it’s ready. If it doesn’t, continue to boil. Test it every 3-5 minutes. It can take up to half an hour to set, depending on how quickly the liquid boils off.
When the marmalade is ready, remove it from the heat and ladle it into the sterilised jars, leaving a little gap at the top. Put circles of grease-proof paper into the top of the jars and leave to cool. Don’t put the lids on until the jars are fully cooled.