Mercedes-Benz of Farnborough

I had the nox sensor in my C- class changed by this company almost 2 months ago when the engine management light care up on the dashboard. After a week, the sign came up again. I called the the emergency breakdown crew who came over to get it checked. After a week, this happened again. This was also resolved by the breakdown team who requested I took it in to the place I got the nox sensor changed. I took my car in two weeks ago to have the servicing and everything done. I was told by this company after the servicing that the vehicle was all good to go and if it happened again I should bring it in. The warning light came up absinthe just 3 days after. I took it in only for them to tell me I have to pay over £500 to get the adblue system changed which they never said just 3 days before. I wasn't treated well and no one really showed any concern for their actions.

I own a 4 years old class C that I bought brand new and that I have regularly serviced at Sandown Mercedes.
During the last service, anyway, they called me saying my MOT failed because of a broken dust cover on the front control arm joint, and that I should have it replaced at the cost of £700, of which £350 just for the replacement part.
I took the car to a normal service center and had the part replaced for less than £100, of which £7 for the replacement part.