Remove the chicken breasts from the water with tongs and place on a plate
Pour the cooking liquor into a large jug
Melt 25g/1oz of the butter in a large heavy-based saucepan over a low heat
Stir in the leeks and fry gently for two minutes, stirring occasionally until just softened
Add the garlic and cook for a further minute
Add the remaining butter and stir in the flour as soon as the butter has melted
Cook for 30 seconds, stirring constantly
Slowly pour the milk into the pan, just a little at a time, stirring well between each adding
Gradually add 250ml/10fl oz of the reserved stock and the wine, if using, stirring until the sauce is smooth and thickened slightly
Bring to a gentle simmer and cook for 3 minutes
Season the mixture, to taste, with salt and freshly ground black pepper
Remove from the heat and stir in the cream
Pour into a large bowl and cover the surface of the sauce with cling ilm to prevent a skin forming
Set aside to cool
Preheat the oven to 200C/400F/Gas 6
Put a baking tray in the oven to heat
For the pastry, put the flour and butter in a food processor and blend on the pulse setting until the mixture resembles fine breadcrumbs
With the motor running, add the beaten egg and water and blend until the mixture forms a ball
Portion off 250g/10oz of pastry for the lid
Roll the remaining pastry out on a lightly floured surface, turning the pastry frequently until around 5mm/¼in thick and 4cm/1½in larger than the pie dish
Lift the pastry over the rolling pin and place it gently into the pie dish
Press the pastry firmly up the sides, making sure there are no air bubbles
Leave the excess pastry overhanging the sides
Cut the chicken breasts into 3cm/1¼in pieces
Stir the chicken, ham and leeks into the cooled sauce
Pour the chicken filling into the pie dish
Brush the rim of the dish with beaten egg
Roll out the reserved pastry for the lid
Cover the pie with the pastry lid and press the edges together firmly to seal
Trim any excess pastry
Make a small hole in the centre of the pie with the tip of a knife
Glaze the top of the pie with beaten egg
Bake on the preheated tray in the centre of the oven for 35-40 minutes or until the pie is golden-brown all over and the filling is piping hot