1. Generously butter a 1.2 litre pudding basin and bring a kettle of water to the boil. Meanwhile, use an electric hand whisk to beat the sugar and butter together in a large bowl until pale and fluffy. Sieve the flour and baking powder together into the mixture, then add the eggs, milk and orange zest. Whisk for 1-2 minutes until smooth and spoonable. Pour into the pudding basin and level the top. Take a double foil square, about 23x23cm, generously butter one side and make a pleat across the middle. Place over the basin, buttered side down, and fold in the edges to seal tightly. Tie in place with kitchen string to ensure no steam can escape. Make a string handle so that it will be easier to lift the hot basin out of the pan.
2. Put the pudding into a large, deep pan and pour boiling water into the pan until it reaches halfway up the side of the basin. Cover with a lid, return to the boil, then reduce the heat to a gentle simmer. Cook for about 2 hours or until the sponge is firm and well risen. Add more boiling water as necessary, to make sure the pan doesn’t boil dry.
3. About 45 minutes before the end of the cooking time, make the orange syrup. Pour the orange juice into a deep, heavy-based pan with 200ml cold water and the sugar and heat gently. When the sugar has dissolved, add the orange slices. Simmer gently for 30-40 minutes, until the orange peel is tender and the liquid has thickened and reduced by about one-third. It should be like a warm golden syrup in consistency.
4. When the pudding is cooked, carefully remove the basin from the pan and leave it to stand for about 5 minutes. Remove the string and foil, loosen the edges of the pudding with a round-bladed knife and turn out onto a serving plate. Spoon over the orange syrup and serve with double cream or custard.