Preheat oven to 200°C (180°C fan).
Clean leeks thoroughly under cold running water.
Slice leeks into thin rounds carefully.
Peel potatoes and dice into small cubes.
Melt butter in large saucepan over heat.
Add sliced leeks to the melted butter.
Cook leeks gently for 10 minutes softly.
Add diced potatoes to the leek mixture.
Pour cream slowly into the vegetable mixture.
Season with salt, pepper and fresh thyme.
Make pastry by rubbing butter into flour.
Add egg yolk and water to pastry.
Roll pastry out on floured work surface.
Fill pie dish with the leek mixture.
Cover filling completely with the rolled pastry.
Brush pastry top with beaten egg wash.
Bake for 35-40 minutes until golden brown.