Preheat the oven to 180°C (160°C for fan assisted ovens)/Gas Mark 4. Lightly grease 2 x 32cm x 22cm Swiss roll tins and line the bases and sides with baking paper.
Stir the cake mix, oil, water and eggs gently together in a bowl, then whisk (by hand or electric mixer) for 2-3 minutes until smooth and creamy. Fold in the lemon zest. Divide the mixture equally between the 2 cake tins, using a palette knife to spread the cake mixture to the corners of the tins and to level the surfaces. Bake for 15 minutes until risen, golden and springy to the touch. Leave to cool in the tins; about 30 minutes.
To make the syrup, place the limoncello, water and sugar in a small pan and heat gently, stirring until the sugar has dissolved. Bring to the boil and boil, without stirring, for 2-3 minutes until slightly reduced and syrupy. Cool for 5-10 minutes.
Turn the cakes out onto large cake boards and peel off the baking paper. Use a teaspoon to drizzle the limoncello syrup over the cakes and leave for 5 minutes.
Spread the vanilla icing in an even layer onto 1 of the cakes, then gently spread over nearly all the lemon curd (reserving 2 tablespoons). Carefully sandwich the cakes together.
Mix the icing sugar and lemon juice in a bowl with the 1 tablespoon water. Beat to make a smooth glacé icing, adding a little more water, if needed. Spoon the reserved lemon curd into a small piping bag fitted with a fine writing nozzle.
Use a palette knife to spread the glacé icing in an even layer onto the top of the cake. Pipe lines of lemon curd at 1cm intervals across the iced cake. Use the tip of a cocktail stick or wooden skewer to drag lightly through the icing at 1cm intervals, and in alternate directions as you go along, to create a feathered effect. Leave in a cool place until set; about 1 hour. Cut into 24 squares and serve. Store any leftover squares in an airtight container for 2-3 days.