Preheat the oven to 180C/165C Fan/Gas 4. Grease a 900g/2lb loaf tin with butter and line with baking parchment.
In a bowl, cream the sugar and butter until pale and fluffy. Add eggs one at a time, beating between each addition. Fold in the flour, a pinch of salt and the citrus zests.
Pour the batter into the prepared tin, smooth the surface and bake for 1 hour. Check the cake after 40 minutes, if it is starting to brown too much cover with aluminium foil for the remaining cooking time. The cake is cooked when a skewer inserted into the centre comes out clean.
Leave the cake to cool in the tin while you make the sugar syrup.
To decorate, cut the lemon and orange rind into very thin strips. Add 100ml/3½fl oz water and the caster sugar to a saucepan over a low heat, stir gently until the sugar is dissolved. Bring to the boil then add the rind to the pan, reserve a few strips of rind for decoration. Cook for about 5 minutes until a syrup forms and the rind is translucent.
Brush the syrup over the still-warm cake and set aside to cool completely. When the cake is completely cold, remove it from the tin.
To make the icing, mix the citrus juices. Mix the icing sugar with enough juice to make a thin icing. Drizzle the icing over the cooled cake and finish with a few thin strips of citrus rind.