Chop and then boil the potatoes in a pan of salted water for 15-20mins, until tender.
Drain the potatoes and allow to steam in the pan for 3-4mins. Mash the potatoes, adding in the milk until smooth and fluffy. Season with salt and pepper. Set aside for use later. - put some in Tupperware to save/freeze for another dish and leave some in pan for use in pie.
Heat the oil in a frying pan and fry the chicken for 5 minutes, or until lightly browned. Add the mushrooms.
Blend the contents of the sachet with the milk and pour over the chicken.
Bring to the boil, stirring continuously.
Reduce the heat and simmer for 8 minutes or until chicken is cooked through, stirring occasionally. Place in a pie dish.
Pile the mashed potato over the chicken, sprinkle with cheese and place under a preheated grill until golden and bubbling.