Cut the bacon into small, pencil-thin strips. Chop off and discard the roots and tough, dark green leaves from the leeks. Slice the rest thinly, no thicker than a pound coin and rinse thoroughly in a colander under running water.
Melt the butter in a deep pan over a low to medium heat, add the bacon and bay leaves, fry for a moment or two then add the leeks, drained but still a little wet.Cover with a piece of greaseproof or baking parchment and a lid.
Remove the leaves from the parsley, keep the stems for stock or soup, then chop the leaves quite finely and set aside.
Wash the clams, removing any with broken shells
In a second pan, deep and to which there is a lid, put the clams together with the sherry, cover tightly and bring to the boil. Let the clams steam for two or three minutes until their shells open. Dispense with any that stubbornly refuse to open.
Transfer the clams to the leeks with a draining spoon, replacing the paper and lid. Turn the heat high under the clams cooking liquor and boil until reduced by half. Pour in the cream, tip in the parsley and season generously with black pepper but no salt.
Remove the lid and paper from the leeks, discard the bay leaves, then gently toss together the clams, leeks and parsley sauce.
Serve in deep bowls, with spoons and paper napkins at the ready.