Preheat the oven to 200C/180C Fan/Gas 6. Roll out the pastry on a lightly floured surface and use to line a 23cm flan or sandwich tin. Prick the base with a fork. Lay a piece of baking paper in the pastry case and weigh it down with ceramic baking beans or rice. Bake for 10 minutes, then remove the paper and beans or rice and bake for another 5 minutes. Set aside. Turn the oven down to 160C/140C Fan/Gas 3.
Pare the zest off 1 orange with a vegetable peeler, then cut into thin strips with a small knife. Put the granulated sugar and 150ml water in a small pan and heat gently until the sugar has dissolved. Add the orange zest strips and simmer for 10-15 minutes or until translucent. Drain and set aside.
Finely grate the zest of another 2 oranges and set aside. Squeeze the juice from all 4 oranges and put in a stainless steel pan. Simmer until the juice is reduced to 125ml. Pour into a measuring jug, add the grated zest, and leave to cool for 10 minutes.
Lightly whisk the eggs and caster sugar in a bowl until evenly blended, but make sure you don't overwhisk and create unwanted air bubbles. Gently whisk in the double cream and reduced orange juice. Pour into the flan case and bake for 35-40 minutes or until just set. Leave to cool.
Sprinkle the chocolate and caramelised zest strips on top just before serving.