There are actually quite a lot of MB indies in that area. I tried HW once, and he was ok. I did try one in Wolverhampton, when I worked there briefly. Cannot remember the name, but they were not good. Charged a lot of money for a very basic service. Claimed to be Mercedes and Rolls Royce specialists, but I am not sure how genuine that claim was. The ones I consistently used were WDM in Perry Barr, who do the job quickly, and will contact you. They will also contact you if anything unexpected comes up, before they go ahead and do it. The number is 0121 3311066. I used them for four years, bought two cars off them, and would still use them if I lived in the area. If you give them enough notice, they will probably give you a courtesy car for the day as well.
The other garage that a lot of people rate is Andy Gayle in Smethwick, but think he's a little bit more pricey. I believe that Andy Gayle, Howard Williams and the lads from WDM all know each other, and did their training at the MB main dealer in Brum, whose name I have forgotten. So they all know their stuff.
I think there is one is Dudley, T.J. Himley if I remember correctly, but I have not used them.