Pull away and discard any tatty leaves, then cut each lettuce into quarters. Wash under cold, running water to remove any grit. Melt the butter in a heavy-based pan which has a lid. Add the diced bacon and leave to stew for 5 minutes or so over medium heat. Trim, wash and cut the leeks into short pieces, add them to the pan and continue cooking, stirring now and again, till they start to soften.
Add the peas, the lettuce, the stock, a seasoning of salt and black pepper and bring the liquid to a boil. Turn it down as soon as it starts to bubble furiously, then cover the pan with the lid. Simmer for 20 minutes, by which time the lettuce will have sunk to four silky mounds and the liquid will have almost cooked away. Lift out the lettuce, peas,and bacon with a draining spoon and place on a serving dish. If there is more than a ladleful of liquid left in the pan, turn up the heat and boil furiously to reduce a little, check the seasoning then pour over the lettuce.