Melt the butter or margarine in a large saucepan. Add the onion, red pepper and potatoes and fry over a moderate heat for 5 minutes, stirring from time to time.
Sprinkle in the flour and cook over a gentle heat for 1 minute. Gradually stir in the stock and bring to the boil, stirring. Lower the heat, cover the pan and cook for 10 minutes.
Stir in the sweetcorn, chicken and milk. Season to taste with salt and pepper, cover the pan and simmer gently for a further 10 minutes until the potatoes are just tender. Taste and adjust the seasoning if necessary. Serve the chowder garnished with the sliced chillies and parsley.