Have the liver at room temperature and put into a flat dish. Sprinkle with salt, pepper and sugar and pour the port and cognac over. Cover and marinate in the refrigerator for two days.
Roll the liver into a sausage shape 8-10 cm thickness and wrap tightly in kitchen cling wrap. Twist ends to seal and tie with string.
Bring a large pot of water to the boil and then remove from the heat. Wait about two minutes and then add the liver sausage and leave until the water is cool.
Refrigerate the liver overnight before using it.