Peel and wash the potatoes and cut into thick slices.
Wash the leeks well, trim and cut into 1" pieces.
Place both vegetables together into boiling salted water and cook until soft.
The water should be as little as is necessary to cook them.
Strain and mash the vegetables, then put them back on the heat and stir in the milk, yoghurt and seasonings.
Whip them over the very low heat until fluffy.
Puree should be softly green.