Place sugar in a heavy saucepan over medium-high heat and cook, stirring constantly, until the sugar melts and turns a golden amber colour, about 10 minutes. Watch carefully once syrup begins to change colour, because it burns easily. Carefully pour the melted sugar syrup into a flan tin. Let cool.
Preheat oven to 180 C / Gas 4.
Pour full fat milk, sweetened condensed milk, eggs, egg yolks, orange juice, orange zest, vanilla extract and cornflour into a liquidiser and blend for a minute or so, until the mixture is smooth. Pour in the cream and pulse several times to incorporate the cream. Pour the mixture over the cooled caramel syrup in the flan tin.
Line a roasting tin with a damp tea towel. Place the flan tin on the towel, inside roasting tin and place roasting tin in the middle shelf of the oven. Fill roasting tin with boiling water to reach halfway up the sides of the flan tin.
Bake in the preheated oven until the centre of the flan is set but still slightly wobbly when moved, 45 minutes to 1 hour. Let the flan cool, then refrigerate for at least 4 hours. To serve, run a sharp paring knife around the inside of the tin to release the flan. Invert a plate on the flan tin, flip the tin over and gently remove the tin to unmould the flan and reveal the syrupy caramel topping.