Put the colcannon, grated potato and flour in a bowl and mix until evenly combined. Add the milk, little by little, until you have a wet dropping consistency. Season to taste with sea salt and ground black pepper.
Heat a non-stick frying pan over a medium–high heat with a little butter and oil. When the butter begins to foam, spoon heaped tablespoon measurements of the mixture into the frying pan and fry for 3–4 minutes on each side until golden brown.
Remove from the pan and place on a plate lined with kitchen paper. Repeat with the remaining mixture. Serve the potato pancakes with fish, eggs, salad, etc.