Indy near East Grinstead
I've used InAutos MB Specialists at Hobbs Industrial Estate, Newchapel, which is just over the border in Surrey (bit scarey I know), a couple of miles North of East Grinstead.
Pedro, the boss has encylopaedic knowledge. I've had them to service both SLK and C200 - uneventful, as services are when they go without nasty surprises, can't remember price but reasonable-ish and all service costs on website. Pedro always willing to discuss issues and good at explaining potential cost implications when dealing with unknowns, to help avoid nasty surprises.
Another time (admittedly Pedro wasn't around), they were unable to rectify electrical fault with indicator in folding mirror which might be reasonable, but then again, maybe not as it's a common enough issue.