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 fish pie.
Mix the Colman’s seasoning mix with milk in a saucepan and stir until it boils.
Add the defrosted fish to the saucepan and poach gently for 4-5mins, then mix in the peas. Place in a pie dish.
Pile the mashed potato over the fish, sprinkle with cheese and place under a preheated grill until golden and bubbling.