Cook the vegetarian sausages according to the instructions on the packet, probably around 20 minutes in the oven.
Meanwhile, heat the oil in a large casserole pot (or, if your pot can't be used on the hob as well as in the oven, start it off in a large saucepan). Add the chopped carrots, celery, onion, garlic and potatoes, and cook over a medium heat for 5-10 minutes, until the onions are fairly soft.
Add the vegetable stock, dark beer, bay leaf and dried thyme, and season with plenty of black pepper.
When the sausages are cooked, cut them into chunky pieces, and add to the stew.
Cover the stew with a lid, and place in the oven at 190°C (Gas Mark 5 / 375°F) for around 1 hour, or until the vegetables are nearly entirely cooked. If you want, cook uncovered for a further 20 minutes or so, until the tops of the vegetables are slightly crispy.
Remove the bay leaf and serve hot.