Pre-heat oven to 200C/400F/Gas 6.
Heat the oil in a pan over a medium heat. Add the leeks and fry for 2-3 minutes, until soft.
Add the flour, stir well and cook for 1-2 minutes.
Remove the pan from the heat and gradually stir in the milk. Return to the heat and cook, stirring well, until the sauce has thickened. Simmer gently for five minutes.
Stir in the prawns and season with salt and pepper.
Place half the sauce into a pie dish. Place the haddock fillets on top, then spoon over the remaining sauce.
Top with the sliced potatoes and season with some more freshly ground black pepper.
Place the dish on a baking sheet and bake for 35-40 minutes, until the potatoes are golden and the fish is completely cooked through.