Wash the cockles really well and discard any with cracked or damaged shells. They should all remain firmly shut, so chuck any away that are gapping open or don’t shut when tapped on the side of the sink. Put in a large bowl and cover with cold water, swish around with your hand and drain. Repeat the process three more times then drain in a colander.
Place a saucepan over a high heat and add the cockles in their shells, with 150ml/5fl oz water and the lemon juice. Cover quickly with a lid and steam the cockles for 4-5 minutes, or until the shells have all opened. Hold the lid tightly and give the pan a good shake every now and then to redistribute the heat. Once the cockles are cooked, tip into a colander and leave to drain.
Melt the butter in a large saucepan and gently fry the onion, celery and bacon with the bay leaf and thyme sprig for five minutes, or until softened and lightly coloured, stirring regularly. Peel the potatoes and cut into roughly 1.5cm/¾in cubes.
Stir the flour and then the potatoes into the saucepan with the vegetables. Slowly add the milk and stock and bring to a gentle simmer. Cook uncovered for 15 minutes, or until the potatoes are very tender and the soup has thickened, stirring occasionally. Pick out and discard the thyme leaf and bay leaf.
While the soup is simmering, take the cockles out of their shells and put them into a bowl. Discard any that haven’t opened. Add the leek, cream and wine to the soup. Return to a gentle simmer and cook for three minutes. Season to taste with salt and pepper. Stir in the cockles and cook for one minute more, or until hot.
Place the bread rolls on a board and cut off the tops with a serrated knife. Pull out the soft bread from inside each roll. Place the rolls on six dinner plates and ladle the soup carefully inside. Scatter the parsley on top and add a young celery stick to each bread bowl. Top with the reserved bread tops if you like. Serve immediately. Eat the soup and the chowder soaked bread, working your way around from the top of the roll.