Put the butter, golden syrup, sugar and marmalade in a saucepan and heat gently until melted.
Remove from the heat and stir in the chopped peel, if using, and the dry ingredients. Add the milk and beaten eggs and mix until smooth. Pour into a 20 cm (8 inch) deep square cake tin, greased and base-lined with oiled greaseproof paper. Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 35–40 minutes until well risen and a skewer inserted into the centre comes out clean.
Meanwhile, put the sliced oranges into a saucepan with the sugar and water. Cover and simmer for 25 minutes until tender. Remove the lid and cook for 5 minutes more, until the liquid has been reduced to about 2 tablespoons. Add the marmalade and heat until melted.
Leave the cake to cool in the tin for 10 minutes then loosen the edges, turn out on to a wire rack and peel off the lining paper. Turn the cake the right way up and spoon over the oranges and sauce. Store in an airtight tin for up to 3 days.