Boil the potatoes in salted water until tender. Drain and mash with the butter.
Poach the fish in lightly salted water or steam it. Cool slightly.
Flake the fish and mix it gently into the potato with the onion, herbs and salt and pepper. Taste and adjust the seasoning then form the mixture into 8 flat cakes. Combine flour with salt and pepper. Lightly coat the fishcakes with the seasoned flour.
Beat the egg with the water and dip the fish cakes first in the egg mixture, then in the breadcrumbs.
Heat a little oil in a heavy-based frying pan and fry the fish cakes over a moderate heat for 2–3 minutes on each side or until golden.
Drain on paper towels and serve with lemon wedges.