Sift the flour into a bowl and rub in the butter until the mixture resembles fine breadcrumbs.

Stir in the caster sugar, then mix to a dough with the egg yolk and a little cold water if necessary.

Wrap in clingfilm and leave to rest in the fridge for 20-30 minutes.

Roll out on a lightly dusted work surface and use to line a 9 inch loose-bottomed flan tin.

Bake blind, covering the base with baking paper or foil and fill with baking beans.

Bake in a preheated oven at 180C for 20 minutes.

Remove the beans and paper and return to the oven for 5-10 minutes until lightly coloured.

Grate the rind and squeeze the juice from the lemons.

Put the juice and rind in a bowl with the cornflour. Add two tbsp from the water and blend to form a paste.

Boil the rest of the water and pour onto the cornflour mixture. Put the mixture into a pan and bring to the boil stirring all the time, simmer for about 3 minutes until it is thick.

Remove from the heat and add the egg yolks and sugar, then return to the stove and heat through for a minute.

Remove from the heat and leave to cool for a minute. Then spoon into the baked pastry case.

Heat the oven to 160 C

Whisk the egg whites until they form stiff peaks.

Add the sugar a teaspoon at a time, whisking in well after each spoonful.

Spoon over the lemon filling spreading right up to the edge of the pastry.

Bake for 30 minutes until the meringue is pale golden brown.