Geoff, be sure that the dealer takes FULL responsability for the repair. Keep a constant eye on how the system performs.
I have to see up to date a W140 that has NOT yet need MAJOR A/C work. It is really a shame. Such a great car with such an unreliable A/C system.
Although, the E class is not excluded. My E320T needed a new evaporator at 22,000 Kms. (Three years old). It seems that the alluminum core installed at the factory rusts very easily. I installed a copper core unit which is guaranteed for life.