Fish chowder

Name: Fish chowder
Prep time: 15 min
Cook time: 20 min
Total time: 35 min
Cuisine: American, British
Ingredients: 1tbsp olive oil; 1 onion, chopped; 2 sticks celery, chopped; 2 leeks, sliced; 1ltr milk; 4 potatoes, cubed; 2tbsp instant thickening granules (we used McDougalls); 350g smoked haddock fillet, skinned; 300g frozen sweetcorn; 2tbsp parsley, chopped (optional)