1) Good luck finding a "good solicitor" these days, and,Speak to a good solicitor.
2) If you thought the garage bill expensive you're definitely not going to like solicitor's bills.
The good news is that you can do this yourself, there is plenty of advice online on taking small claims actions and you certainly do have a valid claim. Neither does the fact that you have paid the bill already make any difference whatsoever, especially given that you took the vehicle to another Mercedes mainstealer. PM me if you need a steer on this....
Edit... That'll teach me to read to the end of a thread before replying, glad to see that you chased them up, let us know how you get on.