Beat the margarine and sugar together in a bowl until pale and creamy. Mix in spoonfuls of the egg and flour until all has been added and the mixture is smooth.
Stir in the lime rind and the juice from ½ a lime. Spoon the mixture into a 20 cm (8 inch) deep round cake tin lined with nonstick baking paper and spread the surface level. Bake in a preheated oven, 160°C (325°F), Gas Mark 3, for 1–1¼ hours until well risen. Leave to cool in the tin then turn out. Peel off the lining paper and slice the cake horizontally into 3 layers.
Beat the butter, icing sugar and remaining lime juice together to make a smooth butter cream. Sandwich the cakes together using the butter cream and jam. Spread the remaining mix thinly over the cake top and sides.
Drape the rolled icing over the cake. Gently press over the top and sides until smooth and trim away the excess icing. Press a small piece of modelling icing into a 6 cm (2½ inch) nonstick cherub icing mould. Invert the mould and gently ease out the shaped cherub. Trim and make another 3 cherubs.
Roll out the remaining icing and cut into thin strips for the icing ribbons. Twist each one like a corkscrew over a wooden spoon handle and leave overnight to dry. Paint gold detail on the cherub wings. Arrange the cherubs on top of the cake, with the icing ribbons. Secure with writing icing.