If Autotrader feel so strongly about this I wonder if they will refuse to let you advertise you car for sale in there publication if you dont have a full main dealer service history.
We all know if they did stop all the shit crappy and stolen cars being advertised there wouldnt be much left and they would go bust.
Anyway when ever I've been to a main dealer it has scared the shit out of me how thick the idiot working on the cars are so much so that the last time I took a car to a dealer ( I needed a job doing that required a special tool I hadnt got and they were going to do the job as i waited ) I stopped them working on my car and had to finnished the job off my self correctly.
Dont get me wrong if you dont know what you are doing get your car fixed by some one that does.