Peel the oranges with a potato peeler and cut each strip into long, thin shreds. Finely grate the zest of the lemon and set aside. Cut the oranges and lemon in half and squeeze out the juice. Reserve 4 teaspoonfuls and put the rest in a small saucepan with the orange shreds. Add the water and bring to the boil, then reduce the heat slightly and simmer for about 30 minutes, or until well softened. Stir 50g of the caster sugar into the pan and boil for another 4 minutes until the liquid is syrupy and reduced to around 3 tablespoons, stirring occasionally. Lightly drain and reserve a third of the orange shreds in a small bowl. Pour the orange syrup and remaining shreds into a large bowl and leave to cool for 10 minutes.
Preheat the oven to 160°C fan/180°C/Gas 4. Line the base of a 1.2-litre nonstick loaf tin with baking paper, so that the liner sticks 2–3cm over the ends of the tin (these will help lift the cake out once cooked). Butter the inside of the tin well.
Put the remaining caster sugar with the butter, flour, baking powder, eggs and lemon zest in a food processor and blend on the pulse setting until just combined, thick and smooth. Transfer to the large bowl and gently fold in the orange syrup, including the orange shreds.
Spoon the cake batter into the prepared tin and level the surface. Bake for 35–40 minutes, or until well risen and pale golden brown. Cool in the tin for 5 minutes, then transfer to a wire rack until cold.
To make the icing, blend the reserved 4 teaspoons of citrus juice with the icing sugar until smooth. Put a plate under the cake while on its cooling rack.
Drizzle the icing over and leave to set for 10 minutes. Scatter the reserved orange shreds on top of the icing and serve in thick slices with tea.