Good indies typically charge around 60-70% of what MB dealers charge.
MB Guildford (in fact, the Sandown Group ... which includes another 5 dealerships) give labour discounts of up to 40% depending on the age of car (plus parts discounts and cheap MOTs), so can actually be cheaper than many indies:
Sandown Platinum Programme The programme that gives you more!
But MB Ascot (closest dealership to me) doesn't discount at all. So you definitely need to shop around.