Preheat the oven to 200C/400F/Gas 6.
To make the pastry, measure the flour and sugar into a bowl and rub in the butter until the mixture resembles fine breadcrumbs.
Add the egg yolk and water and work to a firm dough - this may, of course, be done in a processor.
Wrap in cling film and leave to rest in the fridge for about 30 minutes.
On a lightly floured work surface, roll out the pastry and use to line an 25cm/11 inch loose-bottomed, fluted flan tin.
Prick the pastry lightly all over with a fork, line with greaseproof paper, fill with baking beans and bake blind in the pre-heated oven for about 20 minutes or until the pastry is pale golden and dried out, removing the paper and beans for the last five minutes.
Lower the oven heat to 150C/300F/Gas 2.
For the filling, finely grate the rinds from the oranges and squeeze out the juice. Put the rind, juice and corn flour into a small bowl and blend together. Bring the water to the boil, then stir into the corn flour mixture.
Simmer gently, stirring, until it forms a thick custard.
Mix the egg yolks and sugar together and stir into the custard. Bring back to the boil, whisking until just bubbling. Remove from the heat.
Allow to cool a little, then pour into the pastry case. Spread the mixture out evenly.
For the meringue, whisk the egg whites in a large bowl until they form stiff peaks. Whisk in the caster sugar, a teaspoon at a time, at high speed between each addition.
Spoon the meringue over the top of the orange filling, and swirl it into peaks.
Bake for about one hour, until the meringue is crisp on the outside and marshmallowy underneath. Serve warm or cold.