In a large heavy pan with a lid, heat the olive oil and sauté the leeks, garlic, bacon (or ham) and marjoram for approx 4 minutes; reduce the heat to low, put the lid on and allow it to sweat until the leeks are tender - approx 10 minutes.
Add the stock, cubed potatoes, canned tomatoes, sweetcorn and chili flakes if using. Simmer, covered until the potatoes are just tender. Give it a good grind of sea salt and black pepper to taste.
Carefully add all the fish and simmer very gently until the the cod (or whatever you're using) becomes opaque, approx 10 minutes. Stir very gently, check the seasoning once more then serve in hot bowls with chopped fresh parsley scattered over, with some warm crusty bread on the side.
Not only does this taste better on days 2 and three, it also freezes well - re-heat very, very gently and be careful not to over-stir or you'll break up the fish pieces.