Chop roughly the onion, leek, carrot, garlic & celery.
Heat the oil in a saucepan and gently sauté the chopped vegetables until they're softened, stirring a couple of times. This will take about 10 minutes.
Wash the tomatoes, cut into quarters and add to the vegetables followed by the tomato purée, bay leaves and sugar. Season to taste and, under cover, simmer for 10 minutes.
Add the stock, bring to the boil, then turn down the heat and simmer gently for 20 minutes (covered) .
Using a liquidizer or hand-held blender process until smooth. Press through a sieve if you like it silky.
Taste and adjust the salt, pepper and sugar, if needed.