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Heat the oven to 200C/180C fan/gas 6. Prick each potato a few times with a knife (to prevent them from bursting) and bake on a baking tray for 1 hr-1 hr 30 mins or until soft when pressed. Cut the potatoes in half and scoop out the flesh into a bowl and mash it with a fork or ricer. Save the skins for another day. While the potato is still hot, mix in the cream and milk to make a smooth mash, season and set aside.
While the potatoes are baking, make the fish pie mix. Put a large frying pan on a medium heat. Melt the butter and cook the leeks slowly for 10 mins until they are sweet and tender, then season and add the nutmeg. Stir in the mustard, then pour in the wine and reduce until there is almost no wine left. Pour in the cream and simmer for about 10 mins until reduced by half. Remove from the heat, and add the chives along with the lemon juice and zest. Taste for seasoning.
Stir the haddock, salmon and prawns into the creamed leeks and transfer to a medium baking dish, then top with the mashed potato, smooth over to cover the fish, and scatter over the cheeses. Bake for 25-30 mins until the cheese has turned golden brown.