Put the unpeeled oranges in a pan with a lid. Cover with cold water, bring to the boil and simmer for 1½-2 hours, or until the skins are soft and the insides broken down.
Drain and, when cool enough to handle, cut each orange in half and remove the seeds. Chop everything finely — skins, pith, fruit — in a food processor.
Preheat the oven to 160C/325F/Gas 3 and grease and line a 24cm loaf tin or a 20cm round cake tin.
Weigh the orange pulp and keep 450g/1lb (discard the rest). Place back in food processor with all the dry ingredients and add the eggs one at a time until you have a smooth batter.
Pour the cake mixture into the prepared tin and bake for 1 hour -1 hour 20 minutes, or until the sides start to come away from the tin. Check the cake after 40 minutes and cover with aluminium foil to stop the top burning.
Remove the cake from the oven and leave to cool on a wire rack. When the cake is cold, remove it from the tin. Dust with icing sugar and decorate with caramelised orange slices