Sift the flour into a mixing bowl and cut the butter or hard margarine into small pieces into the flour.
Rub the butter or margarine into the flour with your fingertips until the mixture resembles fine breadcrumbs.
Add cold water a little at a time until a firm dough is formed. Knead lightly to bring the dough together to smooth dough then wrap and chill for 30 minutes if possible.
Use the pastry to line a 25 cm loose bottomed fluted flan tin.
Line the pastry with baking paper and fill with baking beans. Bake the pastry ‘blind’ for 20 – 30 minutes until dry and just colouring.
Reduce the heat in the oven to 150 ° c /Gas mark 4.
Place the cornflour, sugar and lemon rind in a saucepan. Pour in a little of the water and blend the dry ingredients to a smooth paste. Gradually add the rest of the water and the lemon juice, stirring thoroughly.
Gently bring the lemon mixture to the boil, stirring constantly until it thickens and is smooth and glossy. Remove from the heat and stir in the egg yolks and extra 25 g butter until it is melted. Set aside to cool.
Place the egg whites in a large mixing bowl and whisk until soft peaks are formed.
Gradually whisk in the sugar a tablespoon at a time until the meringue is glossy and firm.
Place the pastry shell on a baking tray and carefully pour the cooled lemon filling into the pastry. Carefully spoon the meringue over the filling taking care to create a seal around the edges of the pastry.
Return to the cooled oven and bake for 20 – 30 minutes until the meringue is crisp and pale gold in colour.