Place the fish in a frying pan and cover with water. Slowly heat the water, without boiling. Cover and cook over low heat until just cooked. Drain, transfer to a bowl and flake with a fork. Add the parsley, dill, lemon juice, capers, gherkin and potato, season, and combine well. Divide into four portions and shape into patties. Dust with flour, then refrigerate for 1 hour.
Meanwhile, make the tartar sauce by mixing the mayonnaise, 1/2 gherkin, 2 teaspoons capers, malt vinegar, 2 teaspoons fresh parsley & lemon juice together in a bowl.
Heat the oil in a large non-stick frying pan. Cook the patties for 5-6 minutes on each side, or until well browned.
Place some lettuce leaves, tomato slices, a fish patty and a quarter of the tartar sauce on each bun.