Melt the butter in a heavy-based saucepan and add the onion, leeks and celery. Season and cook gently until soft (about 20 minutes).
Add the milk and cream and bring to the boil.
Add the potatoes and simmer for about 10 minutes.
Liquidise the soup and pass it through a strainer.
Taste for seasoning and add cream if necessary to adjust the consistency.
For the Coddle Mix Poach the sausages and bacon in a medium-sized saucepan of simmering water until cooked. In a separate pan blanch the potatoes, carrot and turnip in boiling salted water until tender but still with some bite.
For the Buttermilk Cream: Semi-whip the cream and add the buttermilk. Season. Whisk until firm.
To Serve: Heat the soup, add the coddle mix and simmer until heated through. Place in four warm soup bowls. Garnish with a spoonful of buttermilk cream and some parsley.