Remove skin and bones from fish. Cut into cubes.
Blend into food processor with salt. Pulse for 20 sec until the fish is roughly blended. Pulse for another 20 sec and add potato flour, pepper and the egg. Gradually add milk while pulsing. Add leeks.
Use spoon to form fish mixture into flat cakes. Heat pan to high heat and add butter. Fry cakes until brown and keep warm in oven.
Melt butter in saucepan and add curry powder. Stir inn flour and gradually add the starchy water and milk. Stir well and let simmer for a few minutes. Salt and pepper to taste. Add chives.