Had the car from new always been serviced and maintained by Mercedes car has done 85,000 and looks like it has just come out of the showroom. Recently took it in for its yearly service and MOT was most surprised to be told the rear subframe had rusted through and it was unfit to drive. The cost of repair would be about £4200. I have had it repaired (not by Mercedes) by a local garage for a total of £1800 including MOT and replacing brake pipes. The old subframe was so badly rusted through in their opinion it should have been replaced two years earlier. I would strongly advise anyone who has a C class with a Chassis no. Starting WDD204 get it checked ASAP.
The dealership offered a reduction of 10% on their price provided they also did the B2 service at a cost of another £1000. Mercedes Benz have not even offered an apology.
Thanks for posting this to alert other owners. As whittingehame has mentioned this is a known fault and Mercedes dealers are covering it. They'll no doubt try to wriggle out of it now that you've had it repaired elsewhere but I'd probably try to pursue this a bit further.