Heat the oven to 190°C (Gas Mark 5 / 375°F).
First, prepare the Yorkshire pudding batter. Sift the flour into a large bowl, and add the eggs and a small amount of the milk. Whisk until smooth, then add the remaining milk a little at a time. You should end up with a thin pancake batter. Season with salt and pepper, and set aside.
Drizzle the oil into a large baking dish (mine measured around 9 inches square), and add the vegetarian sausages. Bake them according to the instructions on the packet. They don't need to be fully cooked - it should be fine to cook them for around 2/3 of the recommended cooking time, as they will continue to cook later. I cooked mine for around 15 minutes.
When the sausages are most of the way cooked and the oil is very hot, remove the sausages from the pan. Pour the Yorkshire pudding batter into the dish, and top with the sausages and prepared vegetables.
Return to the oven and cook for a further 45 minutes, or until the Yorkshire pudding is browned and well risen. Cut into quarters to serve.