Preheat the oven to 180°C, fan 160°C, gas 4. Cook the pasta in a pan of boiling water for 2 minutes less than the pack instructions, then drain, rinse in cold water and set aside.
Meanwhile, melt the low-fat spread in a large frying pan over a medium heat and fry the sliced leek for 2-3 minutes. Add the flour, stir well, then gradually stir in 200ml milk. When it has thickened, add the spinach to the sauce and stir until it has begun to wilt. Stir in half the cheese, some freshly ground black pepper, the remaining milk and the cooled pasta. Leave to cool completely before stirring in the fish pieces.
Spoon the mixture into a 2-litre baking dish and top with the remaining grated cheese.
Cover the dish in kitchen foil. Bake for 25 minutes. Remove the foil and continue cooking for 10 minutes or until the sauce is bubbling and the fish is cooked through.