Preheat the oven to 400°F or Gas Mark 6 and grate the raw potatoes with the larger part of a box grater and place the grated potato into a bowl of cold water to stop them from going brown. Grate the cheddar using the same side of the grater and set aside in a bowl.
Place the smoked haddock and the salmon in a saucepan, pour over the milk and drop in the onion, bay leaf and peppercorns. Cook over a low heat for five minutes, then drain, holding on to the milk but discarding the onion, bay leaf and peppercorns. Then add the fish to your roasting tray.
Melt 2oz of butter in a saucepan then stir in the flour and cook for five minutes to make a roux. Add the milk that you poached the fish in as well as the mustard, and beat, until the sauce is smooth and thick. Season with salt and pepper, add in half the dill and pour over the fish.
Drain the grated potatoes and pat dry with a clean tea towel until they are completely dry. Mix the grated potatoes in a bowl with the grated cheddar, the rest of the dill and sea salt and scatter over the fish and the sauce.
Melt the remaining butter in a saucepan and brush over the top of the pie to ensure the top gets really crispy. Bake in the oven for 20 minutes, or until golden brown.