Boil the potatoes in a saucepan with enough water to cover for 15–20 minutes or until tender. Drain well, then mash.
Meanwhile, place the fish in a shallow pan with enough cold water to cover and add half the lime juice. Bring to the boil, reduce the heat to low and simmer for 1 minute. Remove from the heat, cover pan and leave to cool for 4 minutes. Drain the fish and flake the flesh with a fork, discarding skin and bones.
Mix together the potatoes and fish with a fork, adding the lemongrass, curry paste, spring onions, garlic, coriander, ginger and the remaining lime juice.
Put the flour onto one plate, the eggs onto a second plate and the breadcrumbs onto a third plate. Take about 1 tablespoon of the fish mixture and shape into a fishcake. First dredge it in the flour, then dip into the egg and, finally, coat with crumbs. Shape and coat the remaining fishcakes in the same way, making 24 all together. Chill the fishcakes for 1 hour.
Heat half the oil in a nonstick frying pan. Add half the fishcakes and cook over medium heat for about 3 minutes on each side or until golden. Remove and keep warm while you cook the rest of the fishcakes, using the remaining oil.
For the dipping sauce, place all of the ingredients in a small pan and heat gently for 1 minute. Do not boil. Mix together the lettuce, cucumber and mint for the salad. Arrange the fishcakes on individual plates with the salad and serve each with a tiny dish of the dipping sauce.