For the meringues, heat the oven to 120°C. Line a baking sheet with baking parchment. Finely grate the zest from the lemon and set aside. In a medium-large bowl, whisk the egg whites to stiff peaks, using an electric whisk. With the beaters still running, whisk in the sugar 1 tbsp at a time, whisking the meringue back to stiff peaks again after each addition. Continue until all the sugar has been incorporated. Briefly whisk the lemon zest into the meringue. Drop the meringue mixture in heaped tablespoons onto the baking sheet, leaving a gap between each. Bake in the oven for 1½–2 hours, or until the meringues have dried out completely and will lift off the parchment cleanly. Remove them from the paper and leave to cool completely on a wire rack. Meanwhile, to make the lemon curd, finely grate the zest of the lemons and squeeze the juice. Put the zest and juice in a saucepan with the sugar and butter. Beat the eggs in a bowl to break them up and add them to the pan. Stir over a low heat until the butter and sugar have melted, then increase the heat. Bring the curd just to the boil, stirring continuously, then remove from the heat. Immediately strain through a sieve into a bowl and leave to cool completely; the curd will thicken as it cools. Once cold, cover with cling film and chill until needed. To make the limoncello sauce, finely grate the zest of half of the lemon and pare strips of zest from the other half. Squeeze the juice from the whole lemon. Lightly whip the cream in a medium bowl. It should only just hold its shape as it will thicken when it is mixed with the acidic lemon juice. Fold the grated zest and juice into the whipped cream, along with the limoncello and about 1 tbsp sifted icing sugar. Stir together, taste, and add a little more icing sugar if necessary. The sauce should be very tangy and the amount of sugar needed will depend on the size of the lemon and the acidity of the juice. Chill in the fridge until needed. To make the needleshreds, cut the strips of pared zest into very fine strips. Put the water and granulated sugar into a small saucepan. Dissolve the sugar over a low heat, using the handle of a wooden spoon to gently agitate it, then add the zest strips and simmer gently for 10 minutes. Drain and set aside to cool. To assemble, break the meringues into fairly small pieces and place a few pieces in the bottom of each of 4 sundae glasses. Place a little of the lemon curd on top of the meringues, then spoon over a little of the limoncello sauce. Continue layering the ingredients until they are used up, finishing with the limoncello sauce. Decorate with the needleshreds and serve.