Smoked haddock and sweetcorn chowder

Name: Smoked haddock and sweetcorn chowder
Cuisine: American
Ingredients: Small knob of butter; 1 large onion, finely chopped; 1 celery stick, finely chopped; 1 leek, washed and finely chopped; 3 medium potatoes, diced; 600ml milk; 200ml chicken or fish stock; 500g skinless smoked haddock, cut into 4cm pieces; 326g can sweetcorn, drained; Fresh flatleaf parsley, finely chopped