1 leek and 2 decent sized potatoes per person
Soy milk/cream (optional)
Cut the potatoes up fairly small, boil in the water until they get to the slightly overcooked stage.
While the potatoes are boiling, fry the chopped leeks in the margarine. Add the seasoning - if you're being really basic just use pepper, garlic also works.
Mash the potatoes a bit - they don't have to be completely mashed, more a lumpy puree. (you will notice that I don't own a blender. They can be useful, but are not essential to life!)
Add the leaks and any residual margarine. Simmer everything until tastes merge.
And of course I came to work with no lunch so am feeling hungry after writing this!