You will need to identify the cause of the coil spring failure. It's possible the anti-corrosion covering has become damaged and this has allowed water and winter road salt to prematurely rust the spring. It's also possible that the spring has become damaged by road surfaces (like potholes) or driving over speed humps at speed. I would suggest you have the car inspected to ensure the other springs are not affected. Mercedes-Benz UK may be willing to contribute towards the cost of the repair if the car has a full-service history. Otherwise, I would suggest you take the car to an independent Mercedes specialist.