the 318i was sold 2 years ago to a very happy family .
bmw dealer tried 2 or 3 times to find leak but failed .
a local engineer tested coolant for gasses and found very small amounts which suggested the manifold seal so he replaced the seal and the car never used any more coolant thereafter .
i do not know exact location of the seal but if your car has one then it will be obvious when the manifold is removed .