1) In a large stockpot bring ½ litre of water to a boil. Add clams, cover and cook for 5 minutes. Uncover, quickly stir clams. 2) Cover and cook 5 to 10 minutes longer or until most of the clams are opened. 3) Transfer the clams to a large bowl or baking dish and strain the broth twice through a fine-mesh sieve into a bowl, being careful to strain out the sand. (You should have about 1½ litres of clam broth. If not, add enough water to bring the volume up to 1½ litres.) 4) When the clams are cool enough to handle, remove them from their shells and chop into 1/2-inch pieces. Set clams and broth aside. 5) In a large heavy pot cook the bacon until crisp and the fat is rendered. Pour off all bacon fat except 2 tablespoons. 6) Add 45g butter, onions and celery and cook until softened, about 5 minutes. 7) Add the garlic, thyme and bay leaves, and cook until the vegetables are thoroughly wilted, about 3 minutes, do not brown. 8) Add the potatoes and reserved clam broth, and bring to a boil. 9) Lower the heat, cover, and simmer for 20 to 30 minutes, or until the broth thickens slightly and the potatoes are very tender. 10) Mash some of the potatoes against the side of the pot to thicken the soup. 11) Remove from the heat, stir in clams and double cream and season with salt and pepper to taste. 12) Set aside for 1 hour, covered, to allow the flavours to marry. 13) Place the pot over low heat, and slowly reheat, being careful not to bring to the boil. 14) Serve hot, garnished with remainder of butter and chives.