Orange Cake with Marmalade

This is from Nigel Slater's book The Kitchen Diaries

175g butter
175g golden unrefined caster sugar (have never seen this in Australia: I used plain caster sugar)
a large orange
3 large eggs
75g orange marmalade
175 SR flour
Frosting:
100g icing sugar
2tbsp orange juice

Icing:
100g icing sugar
2tbsp orange juice

Set the oven at 180 degrees. Line a loaf tin about 25x11x7cm deep. Put the butter and sugar in a cake mixer and beat until pale and fluffy. Finely grate the orange zest. Break the eggs into a small bowl and beat them lightly with a fork. With the machine set at moderate speed, pour in the beaten egg a little at a time, beating thoroughly between each addition. Beat in the marmalade and the grated orange zest.

Remove the bowl and fold in the flour with a large metal spoon. Do this slowly, firmly but carefully, until there is no sign of any flour. Lastly, gently stir in the juice of half the orange. Spoon the mixture in the lined tin, lightly smoothing the top. Bake for 40 minutes, checking it after 35 with a metal skewer. (My cake took a little over an hour in the end). Leave to cool in the tin – it will sink slightly. Then remove and cool completely on a wire rack.

Sift the icing sugar and mix it to a smooth, slightly runny consistency with as much of the remaining orange juice as it takes – probably just under two tablespoons. Drizzle the icing sugar over the cake, letting it run down the sides, and leave to set.