I was warned about Sytners in SC by friends when I first got my BMW and have thus only ever bought parts from there. My servicing has all been done by Sytners in Coventry, which is easy to get to from Tamworth as you dont need to go through Cov, and I have found their service to be excellent.
Also I can recommend the Sytners body shop in Oldbury.
Also have you tried Marques on the old A5 in Fazeley. Lots of recommendations from my work colleagues using independent BMW servicing.