My guess is they put it through the car wash, hoovered it out, bit of tyre dressing and straight onto the approved used forecourt...Maybe. Although I suspect not because if they couldn't fix it for me as the first owner after having the car for months on end I doubt they would be fixed for any one else.
Interesting that it's turned up in Newcastle and not on offer at Northampton. Must say it's very disappointing that a c. £100k Mercedes was not properly built in the first place and that your experience of customer service has been somewhat less than acceptable. That is quite different from my/our experience, but we have not yet had cause to 'do battle' with our dealer and the cars we've had have been from rather lower in the MB food chain. Nevertheless we have always so far had excellent customer service from MB.