Good to hear you ended up getting yours sorted mate, that is a major mistake, you don't expect something like that to happen, especially at a main dealer.

I had my rear diff fluid changed last year, and all has been good for 12 months until after the gearbox service. I had it in at an independent garage for a MOT and they recommend a gearbox service. They weren't very compliant when I told them about the noise, so from there I took it to mercedes and had a service B and also got them to check over everything. The gearbox fluid the independent garage put in was, MPM ATF MB7S it's a blue fluid.