Bad service

Very disappointed with mercedes benz Bedfordview my car has been with them for 3 months and it's a return job it was showing handbrake sign I paid 36k for the issue to be be resolved and it kept popping. I took the car back on my there the car started misfiring and they said it could be spark plugs without doing a diagnostic now the spark plugs and coils have been changed but the car is still misfiring they say they think they think the pistol is damaged but how when you haven't even done a pressure test or do a diagnostic the service manager says they have to strip first that sounds like guess work to me such and expensive car and you doing gues work. Another Customer is also complaining about his GLC at the same dealer it seems to me theybjust charge without knowing what th exact problem is. Mercedes benz bedfordview is really terrible 😢