I've always used George Fraser and found his service to be excellent. Though he, or rather his garage, has been getting mixed reviews from Forum members of late. If you do a forum search for George Fraser you'll see plenty of threads.
If its any help, I live in Richmond and used to use George but I have now lost confidence and drive all the way up to Andy Gayle's in Birmingham for a service.
There is one independant in Richmond who specialises in MB and another in Teddington. I've never tried either although my car had been religiously serviced by the one in Teddington by its former owner.
If you want I can check the service book and get you the info.