Preheat the oven to Gas 6, 200C. Lightly oil a Pyrex® glass flan dish.
Roll out the pastry on a lightly floured surface and use to line the dish. Bake blind for 15 minutes, remove the baking beans and reduce the heat to Gas Mark 4, 180°C, cook for a further 5 minutes. Remove and set aside.
Mix the 6 tbsp cornflour with the water in a medium pan. Add the lemon & lime rind as well as the juice and bring slowly to the boil, stirring continuously until thickened.
Remove from the heat, cool slightly then beat in the sugar and egg yolks. Pour into the pastry case.
Whisk the egg whites until very stiff, then whisk in half of the 150g sugar and the cornflour. Fold in the remainder of the sugar and pile the meringue on top of the filling.
Bake in the oven for 20-25 minutes until crisp and golden.