Boil the potatoes for 15 minutes until just tender. At the same time, put the smoked haddock in a deep frying pan, cover with boiling water and simmer for 2 minutes. Turn off the heat and leave to stand for 5 minutes. When the potatoes are done, drain them well and mash.
Lift the fish from the water. When it’s cool enough to touch, remove the skin and bones, then break the fish into medium-sized flakes.
Put all the ingredients, except the flour and oil, in a large bowl. Season and mix well. The mixture should be quite light and may be a little sticky. Sprinkle the flour on a plate, divide the mixture into four and shape the fish cakes, coating them lightly with the flour. Put on a plate, cover with cling film and chill in the fridge for 20 minutes or until needed. (You can make them to this stage up to 24 hours ahead, or freeze them for up to a month.)
To cook, heat the oil in a large heavy-bottomed frying pan, then put in the fish cakes, spacing them well apart. Fry for 5-6 minutes, then turn and cook the other side for about 4-5 minutes. (If you’ve frozen them, fry from frozen for up to 10 minutes longer.)