Cube 200g of the butter and pulse in a food processor with 400g of the flour, until it resembles fine breadcrumbs.
Pulse in the icing sugar and 2 eggs, then bring it together with a splash of milk.
Form the dough into a ball, wrap in clingfilm and chill in the fridge for 20 minutes.
Preheat your oven to 180ºC/gas 4. Grease a 30cm pie dish with butter.
Roll out the dough to a 40cm circle, then place into the pie dish, firming it in place.
Prick all over with a fork, cover with greaseproof paper, fill with baking beans and pop in the oven to blind bake for 10 to 15 minutes, or until cooked.
Crack the remaining eggs into a bowl with the vanilla, cornflour, milk and yoghurt. Add 200g of the caster sugar, 1 tablespoon of the remaining flour and 1 pinch of sea salt. Whisk until combined and leave to one side.
Drain and arrange the peaches in the bottom of the baked pastry case, then pour over the custard.
Return to the oven and bake for about 40 minutes, or until the custard is set to the touch but still has a slight wobble. Cover the pastry edge with foil if it colours too quickly.
Make the crumble topping. In a bowl, rub the remaining 25g of butter and 90g of flour with your fingers until it resembles breadcrumbs, then stir in the remaining 75g of sugar.
Sprinkle over the pie and return to the oven for a further 15 minutes, or until golden. Leave to cool completely before dusting with icing sugar and serving.