Place half the sugar in the bottom of a 23cm tube cake tin; place tin in preheated oven briefly to allow sugar to caramelise.
In a saucepan heat the milk, orange zest and vanilla slowly over low heat, stirring all the time and not allowing the mixture to boil.
In a large bowl, beat eggs with remaining sugar. When milk is hot, pour it slowly and beating constantly, into the mixture of eggs and sugar. Pass the mixture through a sieve into a large jug; pour into the tube tin.
Place the tube tin in a deep roasting tin; pour boiling water into the roasting tin until it reaches halfway up the sides of the tube tin, creating a water bath. Cook for about 30 minute. Carefully remove the flan and invert onto a dessert serving plate, pouring any sauce from the tin on top. Allow to cool before serving.