Preheat your oven to 220°C (425°F fan). Place the sausages in a metal roasting tin, drizzle with sunflower oil, and stir to evenly coat. Insert the dish into the oven and roast for 10 minutes.
Meanwhile, combine plain flour and a pinch of salt in a bowl. Add the eggs and whisk until a dense, smooth batter forms. Incorporate one-third of the milk, mixing thoroughly to eliminate lumps. Add the remaining milk and water, whisking until the batter is entirely homogeneous and fluid.
Carefully remove the hot dish from the oven. Immediately pour the prepared batter into the tin around the partially cooked sausages. Promptly return the tin to the oven, reducing the temperature to 200°C (400°F).
Bake for 30 minutes undisturbed. Refrain from opening the oven door for at least the first 25 minutes to ensure the batter rises properly. Once risen and golden, remove and serve at once.