To make the caramelised oranges, put the caster sugar in a frying pan in an even layer and heat until it starts to melt and turn a golden colour. Tip the pan from side to side to keep the caramel as even as you can.
Once it reaches a dark gold, carefully add half the orange juice from the cake oranges (it will splutter so stand back). Keep on the heat, stirring so that any lumps melt back into the caramel. Add the orange slices and heat gently for about 5 minutes until they soften a little. Lift out and drain, keep the caramel and orange slices for later.