Put the dices potatoes into a saucepan, cover with water and bring to the boil. Simmer for about 15 minutes until tender. Drain, then put them back into the saucepan. Cover and keep warm.
Meanwhile slice the leeks and shred the cabbage.
In a frying pan melt the butter and soften the leek for about 5 minutes. Add the cabbage and simmer under cover for another 4-5 minutes.
Add half of the milk to the potatoes. Mash with a hand-masher (not a blender!) and add more milk if needed to get the right consistency. Warm it through, if needed.
Fold in the cabbage and leek mixture, season with salt and pepper and stir well with a wooden spoon.
Serve warm.