Chop the leeks up very finely and then cook them in melted butter in a pot. The leeks only need to soften and not colour. After about 2 to 3 minutes add the diced potatoes, stir for another 1 - 2 minutes then add the white wine vinegar.

Let this boil away until the alcohol has dissolved. Add to the pan about 100ml of water and the double cream and stir in. Take the oysters and remove from the shell. Chop the watercress finely and add to the pot with the leeks. Cook for 2 minutes and then add the oysters to the soup .

Squeeze lemon juice over the soup and season with salt and pepper. Garnish with chopped chives and finally before serving add the butter and stir until it is dissolved into the soup.