In a large frying pan sauté the leeks in butter until just tender, remove with a slotted spoon, add the chicken and cook until sealed on all sides. Add mushrooms and continue cooking for a further 4 -5 minutes.
Stir in wine and bubble over a medium heat until the liquid is reduced to a third, remove from the heat and stir in leeks, crème fraiche, a few thyme leaves and seasonings. Allow to cool.
Preheat oven to 200ºC (180º for fan assisted ovens)/ 400ºF/ Gas Mark 6.
Take the pastry sheets and cut 12 x 16cm discs and 12 x 11cm discs and lay the large pastry discs on a lined baking sheet. Divide the cooled chicken mixture between them, spreading into an even layer, leaving a border round the edge of each approx. 2cm.
Brush the borders with beaten egg, and lay the smaller pastry discs over the filling.
Bring up the pastry borders and pinch into the top pastry to form a seal, brush again with beaten egg glaze. Make two small steam holes in the top of each pie and bake for 18 – 20 minutes until pastry golden brown. Sprinkle with sesame seeds and a little thyme and serve.