1. To make the crumb crust, place the digestive biscuits in a polythene bag, then roll into fine crumbs with a rolling pin.

2. Melt the margarine in a small sauce pan, add sugar and biscuit crumbs and stir well.

3. Turn biscuit mix into a 23cm/9" flan dish. Flatten to the bottom and sides of the dish with the back of a metal spoon.

4. Make the lemon filling by putting lemon zest and juice into a bowl with cornflour. Blend to a smooth paste.

5. In a medium saucepan bring half(300ml), a pint of water to the boil. Pour in the cornflour mixture. Simmer for three minutes until thick. Remove from the heat and add the egg yolks and sugar. Return to the heat and cook for one minute. Spoon over the biscuit crust.

6. Preheat the oven to 170/325/GS3.

7. Beat egg whites until soft peaks form. Add caster sugar, one tablespoon at a time. Beat until mixture forms glossy peaks.

8. Spoon topping over the lemon filling. Bake for 30 minutes.