For the soup, heat the oil and butter in a large saucepan and gently fry the onion and celery for 6-8 minutes, or until soft. Add the bay leaves and thyme to the pan.
Meanwhile, bring 570ml/1 pint water to a simmer in another large saucepan. Add the clams to the pan, cover with the lid, turn the heat to medium and steam for 2-3 minutes, or until all the shells are open.
Drain the clams, reserving the liquor. Discard any clams that have not opened, then remove the clam meat from the shells and set aside.
Add the sweetcorn kernels, clam liquor and chicken stock to the onions, and cook for 10-12 minutes, or until the corn is soft.
Add the potatoes, and cook for a further 8-10 minutes, or until soft, but not falling apart.
Stir in the clams, cream and parsley, and cook for 4-5 minutes. Season, to taste, with salt and freshly ground black pepper.
For the breadcrumbs, pulse the bread to rough breadcrumbs in a food processor (or roughly tear with your hands). Heat the oil in a frying pan over a medium-high heat, and fry the breadcrumbs with the lemon zest and thyme for 2-3 minutes, or until golden-brown and crisp. Season, to taste, with salt and freshly ground black pepper.
To serve, divide the soup into 4 bowls and sprinkle over the thyme breadcrumbs. Heat the remaining oil in a frying pan and fry the pancetta slices for 2-3 minutes per side, or until golden-brown and crisp, and serve on top of the soup if desired.