Preheat the slow cooker if necessary; see the manufacturer's instructions. Oil a 900 ml (1½ pint) rectangular heatproof dish and line the base and two long sides with nonstick baking paper, checking first that it will fit in the slow cooker pot. Cover the base with sage leaves, reserving the remainder.
Heat the oil in a frying pan, add the onion and fry until softened. Transfer to a bowl and add the sausages, bacon and diced pork. Chop the drained chicken livers, discarding the white cores. Add to the bowl with the egg, vinegar and plenty of salt and pepper. Mix together, then spoon half the mixture into the dish and press down firmly.
Arrange more sage leaves over the pâté, then cover with the remaining mixture. Press down firmly and arrange any remaining sage leaves on the top. Cover loosely with foil and stand the dish in the slow cooker pot. Pour boiling water into the pot to come halfway up the sides of the dish. Cover with the lid or foil, and cook on high for 5–6 hours or until the meat juices run clear when the centre of the pâté is pierced with a knife.
Lift the dish out of the slow cooker pot using a tea towel, pour off the excess fat, stand the dish on a plate, then cover the top with a second plate and weigh down with measuring weights. Leave to cool, then transfer to the refrigerator overnight.
To serve, remove the weights, plate and foil. Loosen the edges of the pâté with a knife, then turn out on to a chopping board and peel off the lining paper. Cut into slices and serve with toasted bread and salad.