Place the strawberries and sugar into a saucepan set over a medium heat and stir until the mixture comes to the boil and the sugar has dissolved. Remove from the heat and pass through a fine sieve, then set aside to cool.
Preheat the oven to 160 C, fan 140C, gas 3. Line a Swiss roll tin with baking paper.
Whisk the egg whites to stiff peaks in the bowl of a stand mixer, then gradually add the sugar until you have a smooth, glossy mixture. Beat in the cornflour and vinegar.
Divide the meringue into three. Leave one bowl white then colour the second bowl light pink and the third bowl a slightly darker pink.
Add the meringue in thirds along the tin then spread the meringue out evenly. Bake in the oven for 25-30 minutes then leave to cool in its tin for 5 minutes. Put a large sheet of baking paper on the worktop and dust with icing sugar. Turn the meringue out onto the paper and peel back the lining paper, then leave to cool.
Mix the roughly chopped strawberries with the elderflower cordial and whip the cream with the icing sugar until fairly firm. Spread with meringue with the cream, leaving a 2cm border free along one short side. Dot the coulis around on top of the cream and swirl into the cream with a cocktail stick. Spoon the strawberries evenly over the cream.chilled, to decorate the top.
Using the paper to help you, roll up the meringue like a Swiss roll, towards the cream-free short side. Leave the meringue wrapped up in its paper and place in the fridge to firm up for 2 hours before serving.
To serve, unwrap the meringue and transfer to a serving platter. Squirt some cream along the top of the meringue and top with the halved strawberries. Serve in slices.