Place the mashed potato, cheese and flour into a bowl and mix well. Using your hands, shape the mixture into two patties.
Heat two tablespoons of the oil in a frying pan. Fry the potato cakes for 3-4 minutes on each side, or until golden-brown.
Place the watercress into a bowl with the remaining tablespoon of oil and the port and stir well.
To serve, divide the dressed watercress among two small serving plates and place the potato cakes on top. Top with the crisp fried bacon.