I use MB Automotive in Loughborough who I find to be excellent, but I don't have an AMG and they definitely target the MB marque rather than specifically try to focus on AMGs.
I see loads of 500s in there in amazing condition. Unless it's something unusual, I would trust them with my AMG if I had one for everything but the most specialist of jobs.
Reasonably priced. Martin and Ed front of house are good chaps.