Place two of the oranges in a deep pan, cover with cold water and simmer for one hour until completely soft. Drain, then transfer to a food processor and blend to a purée.
Preheat the oven to 170C. Grease and line a 23cm loose-bottomed cake tin.
Scatter a tablespoon of the sugar over the bottom of the tin in an even layer.
Using a sharp knife, carefully remove the peel and pith from the remaining oranges, then slice into 1cm-thick rounds, discarding any seeds.
Arrange the slices, overlapping, in concentric circles around the base of the cake tin and put aside for later.
Combine the dry ingredients in a large mixing bowl.
Beat in the eggs, then pour in the oil and stir in the orange purée, making sure there are no lumps.
Pour the batter over the oranges, then sit on a tray and place in the middle of the oven for 75-90 minutes, until a skewer comes out clean. Allow to cool in the tin before transferring on to a plate and serving