Preheat the oven to 190C/375F/Gas 5. Grease the base of a 20cm (8 inch) springform cake tin and line the base with parchment paper.
Put the whole oranges in a pan and cover with cold water. Place a lid on top and bring to the boil, then turn down the heat and simmer for two hours. Check the water during this time to ensure they don't boil dry, adding more if necessary.
After two hours, remove from the heat and leave to cool. (This can be done in advance and then stored in the fridge until needed.)
Remove the oranges from the water. Cut two of them into chunks and remove any pips.
Thinly slice the remaining two oranges and lay half of these on the bottom of the cake tin. Keep the remaining slices for the top of the cake.
Place the chunks of orange into a food processor with all of the remaining ingredients and blend until combined.
Spoon this mixture into the cake tin, covering the orange slices. Top with the remaining sliced oranges and bake in the oven for 45 minutes. Keep a careful eye on the cake during cooking - you made need to reduce the temperature a little depending on your oven.
After 45 minutes the cake should be soft and moist. The usual test of inserting a knife to see if it comes out clean doesn't apply to this recipe.
Let the cake cool slightly in the tin then remove it and leave to cool on a wire rack.
Serve in slices.
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