Place the cockles in a large pan with 150ml/5fl oz of the water. Cook over a high heat, occasionally shaking the pan, until they have opened.
Tip into a colander set over a bowl to collect the liquid and leave them to cook a little.
Melt the butter in a large pan, add the bacon and cook until it is just beginning to brown.
Add the leek, celery and tomatoes and cook until the mixture begins to flop.
Meanwhile, remove the cockle meats from the shells.
Pour all but the last tbsp or two of the cockle liquor into the pan, add the rest of the water and the potatoes and leave the soup to simmer until the potatoes are soft, about 10 minutes.
Add the shelled cockles and taste the soup for seasoning. Whisk the lemon juice with the eggs in a bowl. Pour on a ladle full of the hot soup, whisk together and then stir the liaison into the soup. Stir it over a low heat to thicken slightly but do not let the soup boil. Stir in the parsley and serve.
