Preheat the oven to 200°C, fan 180°C, gas mark 6.
Boil the potatoes in a pan of water until tender, then mash with a third of the butter and a little of the milk. Season to taste and set aside.
In saucepan, bring the rest of the milk to a simmer, add the bay leaves, peppercorns and white fish, then cook for 5 minutes. Strain the fish and liquid through a sieve, keeping the poaching milk.
Flake the cooked fish into an ovenproof dish, and sprinkle over the prawns and eggs.
To make the sauce, melt remaining butter in a saucepan then stir in the flour to form a smooth paste. Slowly add 300ml of the reserved poaching milk, and keep stirring until smooth (add a little more of the liquid if needed).
Season the sauce and stir in the parsley. Pour over the fish in the baking dish and top with the mashed potato. Spread the potato gently over the fish with a fork (for a rough top).
Sprinkle with the cheese, and cook in the preheated oven for 20 minutes until golden. Serve immediately, with some steamed green vegetables if desired.