Peel and roughly slice the carrots. Slice the celery. Peel and roughly chop the onions. Cut the ends off the leeks, quarter them lengthways, wash them under running water and cut them into 1cm slices. Peel and slice the garlic.
Place a large pan on a high heat and add 2 tablespoons of olive oil. Add all your chopped and sliced ingredients and mix together with a wooden spoon. Cook for around 10 minutes with the lid askew, until the carrots have softened, but are still holding their shape, and the onion and leeks are lightly golden.
Peel the potatoes and cut them into 1cm dice. Put the stock cubes into a jug or pan and pour in 1.8 litres of boiling water from the kettle. Stir until the stock cubes are dissolved, then add to the vegetables. Add your potatoes. Give the soup a good stir and bring to the boil. Reduce the heat and simmer for 10 minutes with the lid on.
Remove the pan from the heat. Season with salt and pepper. Serve like this or pulse until smooth using a hand blender or liquidizer. Divide between your serving bowls.