For the cakes: preheat the oven to 180 degrees, and grease and line two 10-inch, two 8-inch and two 6-inch cake pans.
Prepare the 3 vanilla cake mixes according to the package directions. Add elderflower cordial and lemon zest to the batter, and divide the batter among the prepared pans.
Bake until the tops spring back to the touch and a cake tester comes out clean. The time will vary between 20 minutes to 40 minutes for each size cake. Allow the cakes to cool completely on a wire rack.
For the elderflower simple syrup: heat the sugar, water, lemon juice, lemon slices and elderflowers in a medium pot over medium heat until sugar has completely dissolved, about 8 minutes, taking care not to stir too often. Remove from heat, strain and set aside to cool at room temperature.
For the assembly: beginning with the largest cakes, place one cake round on a cake decorating stand. Trim the cake flat, then brush the cake with the elderflower simple syrup and pipe a ring of icing around the top side of the cake. Fill the centre top of the cake with lemon curd and top with the next cake, already trimmed and brushed with simple syrup. Place about 200 grams of buttercream onto the top of the cake and ice around the top. Generously ice the sides of the cake, then with the use of a pastry scraper, scrape the buttercream away to create a naked cake look. Repeat with the remaining 2 tiers of cake.
Place 4 dowels, cut to the height of each cake tier in the centre of the largest tier. Place the 8-inch tier on top. Repeat with the final tier. Decorate with meringues, sugar flowers, fresh flowers and lemon slices as desired.