1. Preheat your oven to 200°C, Gas Mark 6.
2. Half fill a saucepan of water and bring to the boil on your hob.
3. Wash your prawns, peel and cook in your boiling water for 3 minutes until pink. Remove from the water and leave to one side.
4. Peel your potatoes, thinly slice and parboil. Do not overcook them, as they will continue to cook in the oven later.
5. Whilst you wait, place a frying pan on your hob and drizzle in your olive oil. Whilst the oil is warming, finely chop your leek. When hot add your leek and fry until soft.
6. Once soft, add your flour to the pan too, and cook for 2 minutes, stirring gently throughout.
7. Remove the pan from the heat and begin to pour in the milk, pour in a small amount each time and stir to ensure no lumps. Once all of your milk has been added return the pan to the heat and continue to stir the mixture until it thickens. Continue to simmer the mixture for 5 minutes.
8. Stir your prawns and smoked haddock pieces into your pan and season with sea salt and freshly ground black pepper.
9. Grab your pie dish and gently pour your mixture in. Then arrange your pre-cooked potatoes on top, ensuring it is fully covered.
10. Add a little more seasoning on top of your potatoes along with scatterings of cheddar, and place in your oven to bake for 40 minutes, until cooked through and golden brown on top.