Pre-heat oven to 350ºF.

Line a pie dish with the sheets of filo pastry and cut edges to make even.

Heat lemon juice, water and grated lemon rind in a saucepan.

Mix cornstarch to a paste with a little water and add to the lemon mixture cooking. Stir and cook until thick and smooth.

Lightly beat the 1/3 cup sugar or sugar substitute with the egg yolks and then slowly mix into lemon mixture.

Pour into pie shell.

Beat egg whites until stiff but not dry, then gradually beat in 1/3 cup superfine sugar until thick and smooth.

Spread meringue onto top of pie making sure to seal the edges with meringue completely. Melt margarine and brush edges of pastry.

Bake in a preheated moderate oven for 15 minutes.