Preheat the oven to 100°C/fan 80°C/gas mark ¼. Wash 3 x 150ml jars with hot soapy water and place in the oven on a cooling rack for 15-20 minutes or until dry. Turn the oven off and leave them in there until ready to use.
Wash the clementines, then halve and squeeze the juice into a large saucepan. Scoop the pulp out of the fruit, wrap tightly in a cheesecloth and add it to the pan. Cut the peel in to 3mm strips and add to the pan along with the lemon juice, juniper berries and 1.3 litres water. Leave to steep overnight.
Put the pan on a medium heat and bring to the boil. Lower to a simmer and cook for 45 minutes, or until the peel has softened. Remove the muslin bag with a pair of tongs and squeeze out any excess liquid before discarding. Using a clean slotted spoon, remove the juniper berries and discard.
Reduce the heat to low and add the sugar. Simmer until the sugar has dissolved, stirring occasionally. Raise the heat to a boil and cook for 10-15 minutes until the marmalade reaches 105°C. If you don’t have a thermometer, test the temperature by putting a drop of marmalade on to a plate and placing in the freezer for 5 minutes. Push the mixture with your finger – if it ripples slightly, it's ready.
Stir the gin through the marmalade and take it off the heat. Leave to cool for 10 minutes, then ladle into jam jars.