Put the potatoes in a large saucepan, cover with cold water and bring to the boil. Reduce the heat to a simmer and cook for 8-10 mins until tender. Drain and leave to steam-dry for 5 mins. Lightly crush the potatoes with a potato masher until slightly chunky.
Heat 1 tbsp of the oil in a frying pan over a medium heat. Fry the garlic, ginger, mustard seeds and curry powder for 1 min. Stir the spices through the potatoes, season with salt and set aside.
Put the cod in a large saucepan over a medium heat, pour over the milk and bring to a simmer. Immediately remove from the heat, cover and set aside for 10 mins to poach. Lift the fish out with a slotted spoon, then remove and discard the skin. Flake the flesh into the potato mixture in large pieces, then fold in with the lemon zest and juice. Shape the mixture into eight fishcakes using your hands.
Put the flour, eggs and breadcrumbs in three separate shallow bowls. Dip each fishcake in the flour, then the eggs, then the breadcrumbs. Set aside on a baking sheet. To freeze, cover and freeze on the sheet until solid, then transfer to freezerproof containers.
Heat the remaining oil in a large non-stick frying pan over a medium heat and heat the oven to low. Fry the fishcakes in batches for 5-6 mins on each side until golden and crisp. Keep warm in the oven as you cook them. Serve with the salad leaves and slaw on the side.