Peel and dice the potatoes then set to boil in a pot of boiling water, lightly salted. Simmer until tender to a knifepoint then drain.
Meanwhile, shred the cabbage and slice the leek in half rings. Warm half of the butter in a small skillet over a medium heat then add the leek. Cook a minute then add the cabbage and cook, stirring, until it wilts.
Add the remaining butter, milk and some salt and pepper to the potatoes and mash. Add some more milk and butter as needed if your potatoes are still a bit dry or if you want them creamier.
Mix the mashed potatoes in with the cabbage, mix through and serve.
Notes
Note, if you want to make this with leftovers, you can still saute the leek and cabbage in a skillet, just do so for less time. For the potatoes, I'd suggest re-heating in the microwave and mash, if not already mashed, with milk and butter as needed.