Cook the sausages according to the instructions on the packet - probably something like 20 minutes in an oven set to 190°C (Gas Mark 5 / 375°F).
Meanwhile, heat the oil in a large saucepan, and add the onion, garlic, mushrooms, celery and carrots. Cook over a medium heat for 5 - 10 minutes, until the vegetables are soft.
Add the remaining ingredients to the pan, and mix well. Simmer over a medium-low heat, stirring occasionally, until the sausages are ready. Add the cooked sausages to the sauce, and cook for another few minutes, or until the sauce has reached your desired consistency.