Place the potatoes in a pan of water and bring to the boil. Cook for 10 mins until tender. Drain, mash and mix with the butter, lemon zest and parsley.
Meanwhile, place the fish and milk in a shallow frying pan. Cover and simmer for 6-8 mins until beginning to flake. Remove, drain and flake into pieces.
Stir the fish through the mashed potato and pat into 8 flattened balls. Dust with flour and set aside.
Heat the olive oil in a frying pan and cook the fishcakes for 5 mins each side, until golden. Serve with freshly cooked peas.