Chop and then boil the potatoes in a pan of salted water for 15-20 minutes, until tender.
Drain the potatoes, then mash, adding milk until smooth and fluffy. Season with salt and pepper. Set aside for use later.
Mix the Colman's seasoning mix with milk in a saucepan and stir until it boils.
Add the fish to the saucepan and poach gently for 4-5 mins, 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.