In a large saucepan of boiling water, cook cabbage until just tender. Remove and chop or blend well. Set aside and keep warm.
Cook potatoes in boiling water until tender. Remove from heat and drain.
Chop leeks, green parts as well as white and simmer them in just enough milk to cover, until they are soft.
Season and mash potatoes well. Stir in cooked leeks and milk. Blend in the cabbage and heat until the whole lot is a pale green fluff. Make a well in the centre and pour in the melted butter. Mix well.