Melt butter in a large heavy based saucepan over a medium heat. Add the shallots to the butter and cook them, stirring until slightly softened. Add the leeks to the shallots, cook for another 5 minutes, stirring. Add the chicken and slightly cook them and then add the stock, herbs and season with salt and pepper. Bring to a boil so that the chicken is cooked through. Then reduce the heat and simmer the soup for another 25 minutes. Remove from the cooker and let it cool for 10 minutes.
Transfer the soup to a food processor and blend until smooth. (you may need to do this in batches). then return the soup to the saucepan and and warm it up over a low heat for 5 minutes.
Pour the cream into the soup and cook for another 2 minutes, stirring it. Ladle into bowls and serve warm.