Boil potatoes in a saucepan till well done. Drain and mash until smooth. Mix in the yoghurt and season well with salt and black pepper.
Cut the fish into bite size pieces. Place in the bottom of an ovenproof baking dish along with the prawns.
Place the leeks and the fish stock in a medium saucepan and cook for 1 to 2 minutes. Sprinkle the flour over and mix well. Cook for a further minute, then add the mustard and beat well. Gradually add the milk, stirring continuously to prevent any lumps from forming. Bring to the boil, allowing the sauce to thicken. Pour sauce over the fish in the baking dish. Allow to cool for 20 minutes, then cover with the mashed potatoes, either using a fork or a piping bag with a large star shaped nozzle.
Top with cheese and bake in preheated oven for 30 to 40 minutes until golden. Just before serving, sprinkle with the chopped parsley.