Preheat oven to 180°C. Line a swiss roll pan with baking paper.
Place eggs in a large mixing bowl. Beat with an electric mixer until frothy. Gradually add sugar and beat until mixture becomes thick and creamy. Stir in water, vanilla, lime zest and juice.
In another bowl, combine flour, almond meal and baking powder, then gently fold into egg mixture. Spread batter evenly into prepared pan and bake in preheated oven until cake springs back when lightly touched (about 15 minutes). Cool 5 minutes. Invert onto a kitchen towel dusted with icing sugar. Gently peel off baking paper. Roll up cake in the towel starting with the short side. Cool completely on a wire rack.
To prepare the gin and tonic syrup place ingredients in a small saucepan over low heat. Stir until sugar has dissolved. Turn heat up to medium and simmer for 5 minutes or until thickened. Using tongs, dip limes slices one at a time into syrup. Allow any excess syrup to drain off before transferring to a wire rack set over a baking tray to cool.
To prepare the filling, make a paste by combining the cornflour with a little coconut cream. Set aside. Place the remaining coconut cream in a small saucepan over medium heat. Bring to a gentle simmer. Remove 2 tablespoons of heated cream and add it to the cornflour paste. Stir until smooth, then pour into the heated cream. Whisk until mixture thickens. Remove from heat and set aside to cool.
Cream margarine and icing sugar with an electric mixer. Slowly add the cooled coconut milk mixture, while the beater is still running. Stir through 1/2 teaspoon lime zest. Refrigerate for at least 30 minutes.
Unroll cake and spread with 1/2 of the coconut cream mixture to within 1.5 cm of edges. Roll up again, then place seam side down on a serving platter. Drizzle cake with gin and tonic syrup, then frost top and sides with remaining coconut cream mixture. Refrigerate, covered, about 1 hour. Top with remaining lime zest and, if desired, a few slices of lime.