Heat the chicken stock in a lidded saucepan. Add the chicken breasts and bring to a low simmer. Cover with a lid and cook for 10 minutes. Remove the chicken breasts from the stock and set aside on a plate to cool. Pour the stock into a jug for using in the creamy leek sauce.
Melt 25g of the butter in a large, heavy-based saucepan over a low heat. Stir in the leeks and fry gently for 2 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 of the reserved stock, stirring until the sauce is smooth and thickened slightly. Bring to a gentle simmer and cook for 3 minutes. The remaining stock can be discarded or used if needed in another recipe.
Season with salt and pepper. Remove from the heat and stir in the cream. Check the seasoning again and add, to taste. Pour into a large bowl and cover the surface of the sauce with cling film to prevent a skin forming. Set aside to cool.
Preheat the oven to 200C/180C fan/Gas 6. Put a baking tray in the oven to heat.
When you’re ready to bake the pie, cut the cooled chicken breasts into 3cm pieces. Stir the chicken and gammon into the cooled sauce. Pour the chicken filling into the pie dish. Brush the rim of the dish with beaten egg, then remove the puff pastry from its packaging and unroll it.
Cover the pie with the unrolled pastry and press the edges together firmly to seal. Trim any excess pastry. I decorated the top of the pie using the excess pastry! My pie dish is 29cm x 23cm, so the width of the unrolled pastry fit perfectly.
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 minutes, or until the pie is golden brown all over and the filling is piping hot.