Seive the flour into a large bowl and add the butter. Lightly rub the flour and butter between your fingertips until it resembles breadcrumbs. Add a few drops of water and begin to combine with a flat knife. Add a little bit of egg yolk and finish combining with your hands.
Then wrap the pastry and put in the fridge for 30 mins.
When chilled, roll out the pastry until it is just larger than the tin and press into the tin. Trim, pierce base with a fork and blind bake for 20 mins. Then remove from the oven and reduce oven temp. to 150oC.
Meanwhile, make the lemon filling. Put the cornflour and sugar into a bowl and add just enough water to mix to a paste. Put 10fl oz of water into a pan with the lemon zest and bring to the boil. Then add to the cornflour and sugar, mixing continuously until smooth.
Beat in the egg yolks, lemon juice, and butter and return the whole mixture to the pan, heating until thickened. Pour into the pastry case.
Whisk the egg whites in a large bowl until they form stiff peaks, then whisk in the caster sugar a little at a time. Spoon the meringue onto the filling and spread to seal the edges of the pastry case. Swirl the top as you like!