I can recommend Terry at Wayne Gates in Harrow, and Olly at Prestige Car Services (PCS) in Horndean. If you are also looking for remap/dyno work, then MSL Performance in Birmingham. For suspension tuning (and also tyres), Tony at Wheels-In-Motion (WIM) in Chesham. All four are forum sponsors.
If you are looking for dealer service, especially if you intend to take-out a Mercedes Benz Service Care Plan, then I recommend Mercedes Benz Brooklands.