Preheat oven to 170 C / Gas 3. Grease and flour a 25cm tube cake tin.
Stir the flour and baking powder together in a mixing bowl.
Cream the margarine with 225g butter and 400g sugar in a large mixing bowl until light coloured and fluffy. Beat in the eggs, one at a time until mixture is light coloured. Stir in 2 tablespoons rum, 2 teaspoons lime juice, lime zest and vanilla extract until thoroughly blended. Stir in the flour mixture alternating with the milk. Pour the cake mixture into the prepared tin.
Bake in preheated oven until a skewer inserted into the centre comes out clean, about 90 minutes.
Meanwhile, make the drizzle by creaming 4 tablespoons sugar with 50g butter in a small saucepan until light coloured and fluffy. Stir in 2 tablespoons lime juice until evenly blended. Bring to the boil, stirring constantly. Remove from the heat and stir in 3 tablespoons rum.
Cool the cake in its tin for 10 minutes. Turn out onto a wire rack or serving plate. While the cake is still warm, use a skewer to prick the top in many places. Pour the drizzle evenly over the top of the cake so it drizzles down all sides. Cool the cake completely before serving.
If you don't have a tube cake tin, you can divide the cake mixture between three 23cm round cake tins, or two 900g loaf tins. Adjust the baking time as needed.
Made this last night and it came out absolutely gorgeous! I halved the quantities in the recipe to allow for a smaller tin (the result still served 10 substantial slices!) and used local Grenadian limes. - 06 Jul 2012