Wash scallops under cold running water. Cut each one in half. Leave to drain on a paper towel.
Wash the leeks and discard the green parts. Slice the white stems lengthwise into narrow strips about 2 inches long. Place in a saucepan with 1-ounce of the butter, a pinch of salt and 8 tablespoons of water. Cover and simmer for 20 minutes. Keep hot.
In another pan melt the rest of the butter and then gently cook the shallots until they are soft. Add the wine, vermouth and scallops. Bring just to the boiling point, then turn the heat very low and simmer for exactly 2 minutes, or less if the scallops are small.
Remove the leeks from their liquid using a slotted spoon and place in a heated serving dish. With the same spoon place the scallops on top. Add the leek juice to the scallop juice and boil until the liquid is reduced to 8 tablespoons. Pour in the cream, return to the boil and bubble for a moment or two. Season with salt and pepper. Pour over the scallops. Scatter with the chopped parsley.