As has already been said, the choice is about more than money.
I'm guessing that your nearest Mercedes dealership is at Bury St Edmunds. If so, it's part of Group 1 Automotive, the same people as the Peterborough branch that I use. I've been using them, under their various umbrellas, for every service on my C350 since buying new from them in 2012. I've never had a problem with them.
I have used the Service Plans throughout for several reasons. By opting for the longest available option (preferably 4 years) I feel protected against annual price increases. The Plan includes all items listed on the service schedules, including the expensive ATF and spark plug changes. With my C350 being at the top end of the models grouped together at the same price in the Plan, I'm gaining from things like getting 6 plugs changed instead of just the 4 in all the other models priced the same.
If you do decide to go down the Indie route, first check with them on their charges for those ridiculously expensive items.
Probably my main reason for sticking with the dealership is convenience. They're much closer to me than any Indie and they always provide a Mercedes courtesy car. If I preferred, they would give me a lift to and from home/work.
There are many excellent sounding Indies around the country, especially those that specialise in Mercedes. For me, it's not worth the hassle of driving all the way to one of them just to save a few bob. But that's my personal opinion. You should go for what suits you the best.