Preheat the oven 180c/160c(fan)
Place all the fish pieces into a saucepan and cover with milk, bring to the boil and then simmer for 15 minutes. Transfer the fish to an oven proof dish and reserve the liquid for the sauce.
Heat 15g of the butter in a frying pan and sauté the mushrooms. Spoon over the fish, along with the cooked mussels, prawns and squid rings.
Add 45g of the butter to a saucepan, heat and stir in the flour. Cook for a couple of minutes, then take off the heat and add the reserved liquid gradually and keep stirring continuously.
Return to the heat and bring to a boil then add the cream, keep stirring for a smooth sauce. Season with salt and pepper. Pour over the seafood and mushrooms in the dish.
To make the mashed potato, add the potatoes in a saucepan of boiling salted water and boil for 15 - 20 minutes. Drain the potatoes from the water and then add a splash of milk and a knob of butter. Mash with a potato masher until smooth.
Pile the mashed potato over the seafood and sauce and bake in the oven for 15 - 20 minutes or until golden brown.