Peel the tomatoes. Cut them into quarters and deseed them in a strainer over a small bowl to keep the juice.
Melt the fat in a saucepan, add the tomato pieces and juice, and the onion. Cook gently until you have a smooth purée. Meanwhile bring the stock to a boil in your marmite. When the tomato mixture is ready, season with salt, pepper and sugar and tip it into the stock, follo