Clean the leeks, removing the tough, dark parts and discard. Chop the leeks and place in a large saucepan with the drained marrowfat peas, lettuce and spinach leaves, chervil, half the butter, salt, sugar, and one-cup of cold water.
Bring to a boil, then lower the heat, cover, and simmer for 40-minutes.
Lightly blend, process or stick blender the contents of the pan (I leave some texture).
Add the remaining 3-cups of water, and stir until it comes to a boil. Remove from heat.
Heat the green peas in a small saucepan for a minute or two.
Add the remaining butter and the heated green peas to the marrowfat peas.
To serve, garnish with a swirl of yogurt and a small parsley sprig.
*I soaked mine for two-days (second day refrigerated) because I couldn't continue with the recipe due to a commitment. Didn't hurt the peas a bit.