Half-fill the base of a steamer with water and bring to the boil. Cook the potatoes in the water in the base for 15 minutes or until tender, and the fish in the top for 8–10 minutes until it flakes when pressed with a knife. Hard-boil 2 of the eggs for 8 minutes.
Skin and flake the fish, discarding any bones. Shell and roughly chop the hard-boiled eggs. Drain and mash the potatoes with the butter, milk and seasoning. Stir the fish, chopped egg and chopped herbs into the mash. Divide into 8 portions and pat into thick rounds.
Beat the remaining eggs in a shallow dish with the measured water. Put the breadcrumbs in a second shallow dish. Coat the fishcakes in egg on both sides, then coat in the breadcrumbs. Arrange on a baking sheet, cover and chill until required.
Mix all the sauce ingredients with a little seasoning. Spoon into a serving dish, cover and chill until required.
When ready to serve, heat half the oil in a frying pan, add 4 fishcakes, cover and fry over a medium heat for 8–10 minutes, turning once, until golden on both sides and hot through. Keep hot in the oven while cooking the remaining cakes in the remaining oil. Serve with spoonfuls of sauce, lemon wedges and a green salad.