Heat the olive oil and butter in a frypan over medium heat, add the pieces of chicken and cook until lightly browned and almost cooked through. Transfer chicken to a plate and set aside.
Add the leek and garlic to the pan and cook over low heat for 2-3 minutes or until softened.
Add the flour and cook for 1 minute.
Add wine and bring to the boil for 1 minute, then pour in stock and cream and cook, stirring, for a further 5 minutes over medium-low heat.
Return chicken to pan with cooked potato, season well, then set aside to cool.
Cut four 18cm squares from the pastry, reserving off cuts. Place some chicken mixture in the centre of each square, then lift up the sides of the pastry to form a parcel, pinching edges together to seal. Cut out leaf shapes from the pastry offcuts and place over the joins. Make a small pastry rose for the centre of each parcel.
Place parcels on a baking tray lined with baking paper. Brush with beaten egg and bake for 25 minutes or until golden.