Trim, peel and roughly slice the carrots and celery. Peel and roughly chop the onions. Peel and finely slice the garlic. Cut the ends off the leeks, quarter them lengthways, wash them under running water and cut them into 1cm slices. Heat 2 tablespoons of oil in a large pan on a high heat, add all the chopped and sliced ingredients and cook with the lid ajar for 10 minutes, or until softened, stirring regularly. Meanwhile, peel the potatoes and cut them into 1cm cubes. Put the stock cubes into a jug or pan, pour in 1.8 litres of boiling water and stir to dissolve, then pour into the veg pan. Add the potatoes. Bring to the boil, then reduce the heat to low and simmer for 10 minutes with the lid on, then remove from the heat. Season to taste with sea salt and black pepper, then either serve like this or pulse until smooth using a stick blender or liquidizer.