For the fish stock, put the oil into a heavy saucepan and heat; then add the vegetables and stir well to coat with the oil. Cover and sweat over a low heat for about ten minutes; do not allow them to colour. Take the lid off the pan and pour in the white wine. Boil for one minute. Now add 1.2 litres/2 pints of water and the smokies. Bring up almost to the boil; then skim well. Reduce the heat and simmer very gently for 20 minutes. Allow to cool. Once cold, strain the stock through a very fine sieve, preferably lined with a double layer of muslin. (The stock can be made two days ahead and kept chilled until needed.)
To make the soup, put a large saucepan over a medium heat. Add the butter and, when melted, add the shallots, leek and garlic. Cover and sweat for 5-10 minutes, but don't allow the vegetables to colour.
Add the cooked potatoes. Flake the smokies and add to the pan. Cover and sweat for a further two minutes, then add 1 litre/1¾ pints of the reserved stock and season with freshly ground black pepper. Bring to the boil and simmer for 8-10 minutes.
Take the pan off the heat and allow to cool slightly. Carefully pour the mixture into a blender and put on the lid. Take the stopper out of the hole in the lid and cover the hole with a tea towel to stop the hot mix splattering. Blend for a few seconds; then gradually add the milk and cream through the hole. When the milk and cream is all added and the mixture is smooth, pass through a fine sieve to give a fine, velvety texture.
To serve, pour the soup into a clean pan and heat gently. Flake the remaining smokie and divide among four warmed shallow bowls. Ladle in the soup and garnish with chopped chives.