For the mashed potato topping, cook the potatoes in lightly salted boiling water for 10 minutes, or until tender. Drain and mash with 2 tbsp margarine or butter and 100ml soya. Season with salt and pepper and set aside.
Meanwhile, to poach the fish, pour 400ml milk into a saucepan and add the fish pie mix and king prawns. Carefully heat the milk, being careful not to let it boil over, and gently simmer for 2-3 minutes, or until the fish is cooked through. Then add the frozen peas to the pan. Remove the fish and peas, reserving the poaching milk, and set aside.
Melt 2 tbsp margarine or butter in another saucepan. Add the flour and a little of the reserved poaching milk and mix into a smooth paste. Gradually stir in the remaining milk, a little at a time to avoid lumps. Simmer, stirring, for 2-3 minutes, or until the sauce thickens. Season with salt and pepper.
Preheat the grill to a high heat.
Spoon the fish mixture into the bottom of an ovenproof dish and mix in the egg. Finally top with the mashed potatoes. Run the prongs of a fork over the surface of the mash to give it a rustic look. Place the fish pie under the grill for about 5 minutes or until the top is golden.
Meanwhile, cook the peas according to the packet instructions. Serve the fish pie with the cooked peas.