1.Preheat oven to moderately slow. Grease deep 22cm-round cake pan; line base and side with baking paper, bringing paper 5cm above edge of pan.
2.Beat butter, rind and sugar in large bowl with electric mixer until light and fluffy. Add eggs, one at a time, beating until just combined between additions. Fold in combined sifted flours and juice in two batches. Spread mixture into prepared pan.
3.Bake cake in moderately slow oven about 1 hour. Stand cake in pan 5 minutes before turning onto wire rack to cool.
4.Meanwhile to make icing; sift icing sugar into small heatproof bowl; stir in butter and enough juice to form a firm paste. Place bowl over small saucepan of simmering water, stir until icing is a spreadable consistency; do not overheat. Top cake with glacé icing.
Icing suitable to microwave. Uniced cake suitable to freeze for up to three months.Note