Chicken and Mushroom pie with buttery mashed potatoes

potato 42.0% , Marinated cooked chicken breast with added water 15.0% , milk 11.5% , mushroom 8.0% , double cream 4.0% , white wine 4.0% , butter 4.0% , onion 4.0% , wheat flour 3.8% , corn flour 1.9% , vegetable oil 0.9% , extra mature cheddar 0.5% , chicken broth 0.2% , salt 0.1% , cep 0.1% , white pepper 0.0% , yeast 0.0% , Colour 0.0% , E100 0.0%