Put the potatoes in a large saucepan of boiling water and cook for about 15 mins until tender. Meanwhile, heat the milk in a saucepan, then set aside. Melt the butter in a medium-sized saucepan over a low heat. Stir in the flour and the mustard powder, cook for 1 min or so, then start to ladle in the hot milk, stirring as you pour, until it’s all added. Cook for a further 10 mins until you have a smooth sauce. Stir in most of the cheddar until fully melted into the sauce. Season with the paprika, cayenne and a little salt and pepper.
Put the potatoes in a small ovenproof dish, spoon over the cheese sauce and sprinkle with the remaining cheddar. Can be prepared a day ahead and kept covered in the fridge. Place in the oven alongside the fish and cook until bubbling and golden.